Problem with period and birth control

Celina

Hi guys I just had a quick question

Yesterday I had sex with my boyfriend and my period was supposed to start yesterday. So after we had had sex there was a lot of blood so I assumed my period had started (I have been having Pms symptoms) and I was told by my doctor to take my first Birth control pill when my period starts so I took one yesterday but no more blood appeared. Only slightly pink brown discharge. Does anyone know what to do?