Late period/possible pregnancy scare

My boyfriend and I were intimate on Saturday March 14/15, we didn’t have sex with penetration it was all oral and some humping (with no clothes). My last period was February 16-18 and usually it comes every month, no skipping. I’m 98% sure I didn’t get any semen in or even on my lady parts but the 2% of uncertainty i have is killing me because my period is LATE (I was supposed to get it 2/3 days after our night together, march 18) and I’ve never had a late period like this. Not sure if it’s late because he did get some if his semen on me and I’m pregnant OR because I’m SUPER STRESSED. I’m so scared because I don’t want to be pregnant. I didn’t buy plan b because, like I said before, I was just so certain we were in the clear. Has anyone else had a similar experience? Or has any advice? I need help remaining calm.