Random pain??

I was sitting on the toilet and I got a random pain in my uterus then it stopped then started again I’m confused on what just happened this has never happened to me before and I don’t have cramps like that I’m on birth control but recently I had took my pills late for 3 days one being the day I had sex with my boyfriend (Saturday) he didn’t pull out

Sorry if I put in the wrong place just didn’t know where