By Lisa C. Bailey
Updated: February 22, 2008

It's one thing to breathe (and sweat) a little more heavily than usual during an intense workout, but it's another thing entirely to find yourself gasping for air during a leisurely after-dinner walk. While occasional exercise aches and pains are rarely cause for alarm, certain symptoms that pop up out of the blue when you exercise could sometimes signal something serious, says Martha Gulati, MD, assistant professor of preventive medicine at Rush University Medical Center in Chicago. If you're otherwise healthy and in good shape but experience any of the warning signs listed below during a workout, stop what you're doing and see your doctor right away.

What You Feel: Chest pain, pressure, or tightness
What It Could Mean: Heartburn, muscle strain, or something more serious, like heart disease

What You Feel: Unusually rapid heartbeat
What It Could Mean: Heart disease, arrhythmia, anemia, thyroid disease, or even just too much caffeine

What You Feel: Difficulty breathing
What It Could Mean: Heart disease or asthma

What You Feel: Fainting
What It Could Mean: Heart disease, arrhythmia, a glitch in your nervous system, dehydration, or low blood sugar

What You Feel: Severe headache
What It Could Mean: Dehydration, stroke, cerebral aneurysm, or high blood pressure