Hello,
The normal menstrual cycle length is 28 days but can vary from 21 days to 35 days and this is considered to be normal. Frequent menses (cycle less than 21 days) can occur due to pelvic inflammatory disease, sexually transmitted disease, endometriosis, fibroids, cervical polyp, birth control pills, stress, exercise etc.
You need to consult a gynecologist to rule out all these causes.
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 and please do keep me posted on how you are doing.
You'll need to get hormone levels and other things checked. Go see your gynecologist.