Hello,
First of all, you should make sure that these clots are not due to any miscarriage by getting your pregnancy status checked. Other causes can be hormonal imbalance(can be due to PCOS),hypothyroidism, cigarette smoking or underlying Chlamydia infection while on birth control, ectopic pregnancy, miscarriage after pregnancy and fibroids and polyps.
I feel that an examination by a gynecologist will be the best.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.
Could it be related to your birth control method? Contact your physician.