Various factors causing it are endometriosis,ectopic pregnancy
imbalance,endometrial hyperplasia,polyps,use of IUD’s and depo shots, sexually transmitted diseases,PCOS and benign and malignant pathologies of the uterus.
You should get a reevaluation done from your doctor.
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.