I am sixteen years old. Two days ago I was having sex with my boyfriend USING A CONDOM. We switched positions and it was still on, but when we moved again a few minutes later, we noticed it had come off inside of me. He didn't bust while he was in me, and I don't think he was even close. I think I was on my last day of ovulation though. He hadn't cum earlier that day, and I'm almost positive that he urinated after the last time he did. I know that precum itself doesn't have sperm in it, but if there is left over sperm it can carry it out.
so here is my question;

How worried should I be about pregnancy? Should I consider the plan-b pill, or are the chances pretty low?