Don't panic if your pair are often perked up even when you're not cold. Being nervous, pregnant, or aroused can make these moody little things spring to life. So can wearing certain clothes. The concern comes in when it's only one nipple that's usually erect, says Rebekah Gee, M.D., an ob-gyn at the University of Pennsylvania. If you notice asymmetry, check in with your doctor, says Dr. Gee. This symptom may be a sign of a benign pituitary tumor, which can typically be treated with medication.