Welcome to HealthcareMagic. This could be due to minor trauma to the vagina or having intercourse with dry vagina. If you had intercourse close to your periods, then it can be due to start of your periods or old blood from periods as well. There is nothing to worry as for now.
If the repeat happens then get it checked from a gynecologist as infections, cervicitis
(inflammation of cervix);vaginitis
(inflammation of vagina);cervical ectropion
(condition in which the cervical tissue is more susceptible to get injured by sex);cervical polyps; uterine fibroid
etc need to be ruled out.
Hope it helps you. Regards.