Passing blood clots during menstruation can be normal. The amount, length and frequency of menstrual bleeding vary from woman to woman.
However, passing large blood clots may be a sign that something's not right. If your periods seem heavier than usual — for instance, soaking through one pad or tampon every hour for several hours, passing large blood clots, or passing many smaller blood clots in a short time — check with your doctor.
Your doctor can determine if there's an underlying problem causing your heavy periods. Your doctor may also recommend a blood test to check for anemia, a blood condition that can cause you to feel weak or tired.