What Causes Abdominal Cramps After Sex?

I just had sex this Saturday morning for the first time ever. Unfortunately, I wasn't thinking, and clearly neither was he, and a condom wasn't used. He didn't cum in me and I couldn't feel any pre-cum which I know is hard to "feel" in the first place. I took Plan B One Step Monday afternoon and ever since then, I have been having mild cramps in my abdominal section. I'm not sure if it's from having sex and if it's a "sign", or if it's the Plan B, but either way... I'm terrified. Help? :(