What’s going on

So my period is due on Jan 12 I had my last period on Dec 15. I have been on the pill for a year and a half now. I had sex on Dec 13. On Dec 30 I woke up to what seemed like my period but throughout the day there was no blood. That same day I had sex. Now Jan 1 I have woken and the same thing.

What could be causing it?