What’s wrong with me?

My boyfriend and I had sex two days ago and for the first time he finished in me. I take my birth control at the same time everyday, if I’m off, it’s by about 30 minutes, so I would say I take it perfectly. However, I’ve been cramping on and off since we had sex and I’m worried that an egg is fertilizing. My period is due in about 3 weeks so I’m not sure what to do.