Plan B? Am I okay to take it?

I am a 15 year old girl. I've been paranoid for less than 48 hours that I may have been impregnated. I say may, because my boyfriend and I were swimming, got undressed around each other. I slipped on wet grass and he laughed, got down, and started kissing me. We were both naked, and for a split second I felt his penis at my entrance. He quickly fixed himself. But for some reason I am horribly paranoid and scared and just want to take a Plan B to be sure. Even though we didn't have intercourse, some say men don't even know they pre-ejaculate, but even he says what I think happened didnt happen. I'm scared. I've never been on birth control or anything, I'm a clean slate. Am I okay to take a Plan, of my sister's, just to calm myself? Will it not work for me? I'm very confused. Help.