Hi, and thanks for your medical question on eHealth forum!
Well the two most common cause of inter menstrual spotting are pregnancy and birth control pills.
Other causes are hypothyroidism, cervical dysplasia, cervical polyps, cervicitis (infection/inflammation of cervix), vaginitis (infection/inflammation of vagina), cervical and uterine cancer, uterine fibroids, hormonal imbalance as in polycystic disease of the ovary, use of blood thinners, genital ulcers, STDs and stress.
If you have missed periods, take the pregnancy test and see.
Since a confirmed diagnosis cannot be given on net please consult your gynecologist once. Hope this helps. Take care!