It's early! It's late! It's heavy! It's AWOL! No worries—we've got answers to all your monthly mysteries.

Getty ImagesAt this point in your life, you're probably pretty tuned-in to what's normal and what's not for your period. So when menstrual weirdness strikes, you might fear that the "off" symptoms are a sign of a bigger health issue. OK, now relax. Odds are there's a simple, nonscary cause. "Your cycle into your 30s and 40s can change for many reasons, like birth control issues, age-related changes or even having a baby," says Alyssa Dweck, MD, an ob-gyn in Mount Kisco, N.Y., and co-author of V Is for Vagina. We asked experts to shed light on five out-of-the-blue symptoms.

"My Cycle Is Suddenly All Over the Map"

When your flow veers from its usual schedule for more than a few months in a row, take note. One possible explanation: perimenopause, the up-to-10-year stretch before menopause sets in. "During this transitional phase, which can start during your 30s or 40s, you're still fertile, but your ovaries begin sputtering out—they sometimes miss the hormone signals sent by your pituitary gland indicating that it's time to cue eggs for ovulation," says Mary Jane Minkin, MD, clinical professor of obstetrics and gynecology at Yale University School of Medicine. Result: Your period may come sooner or later, or you might skip a cycle entirely. Perimenopause isn't itself a medical condition needing treatment, but symptoms (including headaches, sleep problems and hot flashes) can be eased with birth control pills, which even out your hormones, Dr. Dweck says.

A newly unpredictable cycle can also be caused by an overactive or underachieving thyroid—the gland in your neck that helps regulate many body functions, including ovulation. Both hyperthyroidism (when your thyroid pumps out too much thyroid hormone) and hypothyroidism (when the gland makes too little) become more common as we get older. Each is diagnosed via a simple blood test, and medication can get things back in order, enabling your periods to return to normal.

"My Cramps Are Worse Than Ever"

If your cramps used to score a 6 on the pain scale and are now topping out at a 10, it could be a tip-off to endometriosis. This condition, which strikes up to 10 percent of women, is due to uterine tissue growing in other parts of the pelvic cavity. When your uterus sheds its lining each month, the wayward tissue tries to slough off, too. Trapped, it sidelines you with killer pain.

Endometriosis doesn't usually come on suddenly—typically, you'd notice a gradual uptick in month-to-month pain in your teens or 20s. But it might appear to hit completely out of the blue if you've recently stopped taking hormonal birth control. "The pill can mask the pain, so a lot of women notice how bad it is only when they go off oral contraceptives," says Ashley Roman, MD, clinical assistant professor in the department of obstetrics and gynecology at NYU Langone Medical Center. Your doc may diagnose it via a pelvic exam and ultrasound; treatment may include hormonal meds that lower estrogen levels and shrink uterine tissue.

Another cramp culprit: uterine fibroids. These benign growths form on or inside the uterine wall; they range in size from a seed to a grapefruit (or larger) and are diagnosed by ultrasound. A whopping 30 percent of women in their 30s and 40s have them. Fibroids sometimes cause period pain; occasionally a woman has chronic discomfort all month long. The good news is, fibroids are unlikely to ever pose a serious threat to your health, but if you're in agony, there are meds and procedures to shrink or remove the growths.

Next Page: "I'm Using Tampons by the Crate" [ pagebreak ]
"I'm Using Tampons by the Crate"

If your flow has been more like a flood for at least two months and you're changing your pad or tampon once an hour or so, you may be able to pin the blame on a fibroid or a polyp—another type of benign mass (it's like a fleshy bulb of endometrial tissue) that can form in the uterus or on the cervix. "Both fibroids and polyps cause heavy bleeding during your period and sometimes spotting between cycles as well," Dr. Dweck says. Like fibroids, polyps get more common as you reach your 30s and 40s, and they, too, are unlikely to become malignant. Still, your ob-gyn might want to do an endometrial biopsy, in which tissue is removed from the lining of the uterus, to rule out uterine cancer, a rare possibility, Dr. Minkin says.

Another reason for copious bleeding may be a miscarriage; it can sometimes take a few cycles for the pregnancy to be expelled, Dr. Roman says. Endometriosis can also cause heavy flow. Or a thyroid on the blink could be to blame. And, yep, perimenopause is a possible factor: When ovulation becomes erratic, progesterone levels decrease, and that can result in a thicker-than-usual uterine lining—and a heavier flow once it's shed.

"I've Started Spotting Between Periods"

Spotting is the "I feel so tired" of the ob-gyn world: It's a symptom with many potential causes, almost all of them benign. First, are you on a new type of hormonal birth control? Low-dose pills and the Mirena IUD can trigger breakthrough bleeding, which typically resolves after three to six months. Hormonal shifts from perimenopause or a thyroid dysfunction are also possibilities; your uterine lining isn't getting a clear go-ahead to shed, so it's doing so at random times. Your MD will probably check for fibroids and polyps, too.

The one time to hightail it to your doctor's office (rather than wait and see for a few months) is if the spotting happens after intercourse. "This could be a sign of infection, inflammation, precancer or cancer," Dr. Dweck says. Your doc will likely do a Pap and further testing to rule out cervical cancer.

"My Flow is MIA"

Missing one or two periods is no cause for alarm. But when the third is a no-show, take inventory. Assuming you're not pregnant, your first thought might be early menopause. But actually, "only 1 percent of women under 40 and 5 percent of women under 45 are in menopause, which is defined as a full year without a period," Dr. Minkin says. A more likely explanation: Perimenopause has put the brakes on ovulation for a few months, but not permanently.

Stress also can be at play—we're talking about the acute kind that stems from a death in the family, a job loss or some other life-changing situation. Shed a lot of weight quickly? A too-low body-fat level knocks your pituitary gland for a loop, halting your cycle. "It's your body's way of preventing you from getting pregnant when you might not be strong enough to carry a baby to term," Dr. Dweck says. If a crash diet or intense workout regimen has caused you to drop significant weight (a BMI of less than 19 is a definite red flag), ease up on your routine so you gain a few pounds back. Once you do, your periods should return to their old pattern.