3 weeks late on period had bleeding for 4 days with some clots, could it be a miscarriage?

Yes..: At your young age, it is not unusual to occasionally have a late period. However, if you have been sexually active around the time of your last ovulation, the bleeding that you describe may be due to miscarriage.

Common problem: Irregular periods are one of the most common problems obgyns treat. If this is one time problem it usually resolves on its own. If this is a recurring problem you should be evaluated. Infections & hormonal changes can cause spotting. If there is a concern for pregnancy then do a home pregnancy test . There are many options available to help regulate the menstrual cycle. Schedule a visit.

Agree with Dr. O: It is likely that you had spontaneous loss of an early pregnancy, which is not uncommon. About 25% of early pregnancies are lost, mostly due to chromosomal defects in the embryo. Usually the only manifestation is a slightly heavy, delayed period. If you do not wish to be pregnant, use contraception all the time, every time. Consider implanted contraceptive, or IUD. Get HPV vaccine.