Intermenstrual bleeding may be due to many causes. If it was after intercourse or masturbation then chances of genital infection or cervical polyps are there.They are tiny, harmless growths on the outer surface of the cervix and can bleed after intense sexual intercourse.Stress and anxiety can also cause intermenstrual bleeding.
Other causes may be uterine fibroids,miscarriage after a pregnancy,ectopic pregnancy,fluctuation in hormone levels,IUD(intrauterine device)use,use of anticoagulant drugs and hypothyroidism.
Please get yourself evaluated by a gynecologist as pelvic examination and investigations are required to confirm a diagnosis.
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.