Causes may be uterine fibroids, miscarriage after a pregnancy, ectopic pregnancy, fluctuation in
hormone levels, genital infections, polyps, IUD (intrauterine device)use, use of anticoagulant drugs and hypothyroidism.
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.