The commonest cause is pregnancy with
impending miscarriage. Few other conditions causing spotting between periods
are endometriosis, STDs, cervical, uterine or ovarian cancers and ovarian cysts. Any vaginal bleeding other than your periods should be evaluated. Consult your gynecologist for more details. You may need pregnancy test, pelvic scan and pap smear. Keep me posted.
Best wishes and take care!