getting my period after sex while on birth control?!?! (not supposed to have period)

okay so basically me and my best friend are both on birth control and we never get our periods, but she told me that whenever her boyfriend would cum inside her, she would end up bleeding for like a week after. and the same thing happened to me!!! but only when my man nuts in me? is that normal?? i haven’t had my period in a year sex or no sex, it’s only when he cums in me. i’m