Sex/period question

Heather

So I got off my period the day before yesterday today me and my boyfriend had sex doggy style which towards the end hurt but I felt fine and then a few minutes later I feel a gush of liquid figured it was seman so I went about my day it happened again and then I went to go pee and my underwear was covered in blood I’m on the pill and I started my white one today anybody know why this happened and I have a bad headache and I’m a little dizzy