Please help..

My SO and I were grinding against each other without any clothes/undies on and his penis accidentally slipped inside of me a bit halfway.. idk how but it just happened, he said he wiped it before we did what we did and of course I was wet. He says that we’re okay and he’s confident that we’re okay but I’m kind of worried about it. My period is irregular and I should start this Thursday. But my question is should I take a plan b? This happened last night around 10pm btw