Chances are, the discharge is nothing to worry about. Stimulation from sex, exercise, or just washing your breasts can lead to some leaking of harmless secretions. Medications like birth control pills and antidepressants can cause leakage, too.
If it doesnt go away in about a week, though, see your doc. Shell check for a cyst, which usually goes away on its own, or an abscess, which would require antibiotics.
Your MD may also run blood tests to check your hormone levels. In particular, shes looking at prolactin, which is produced by your pituitary gland. While its primary function is to promote lactation in breast-feeding women, non-nursing women make it, too, and an increased sensitivity to prolactin can lead to a little milk production and leakage.
Your level may climb during times of physical or emotional stress or after a breast exam. To keep a leak from showing, try nursing pads or a thicker, more padded bra for a few days.
A few caveats: if the discharge happens with no stimulation, if only one breast is leaking, or if you see blood, see your doctor immediately. These may be signs of breast cancer.