It can be due to metrorrhagia(heavy periods). Various factors causing it are endometriosis,ectopic pregnancy,
hormone 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.