Hi! Many thanks for your question on eHealth forum!
Well the two most common cause of inter menstrual spotting are pregnancy and birth control pills. However since it has happened just after periods, chances of pregnancy are less. It could be due to ovulation.
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.
Since a confirmed diagnosis cannot be given on net please discuss these possibilities with your gynecologist once. Hope this helps. Take care!