Hello,
You have not mentioned your age but in premenopausal women, it is usually due to ovarian cysts, ectopic (tubal) pregnancies, benign (noncancerous) or malignant (cancerous) tumors, endometriomas, polycystic ovaries, and tubo-
ovarian abscess. In post menopausal women, it may be caused by cancer, fibroids, fibromas, diverticular abscess.
Get it checked from a gynecologist to get further investigated to determine the cause.