• Period right after intercourse?

Hi. I had unprotected sex last night (about 23 hours ago). In the morning (like 4 or 5 hours after) I noticed a little blood spotting but that was all. At night, like 16 or 17 hours after intercourse, the spotting got heavier like a period. I also have cramps so honestly it just feels like I got my period, I'm even wearing pads, but I'm afraid it's something else. Also, it does look a little more watery than my normal period. It was my first time with a guy. We used the withdrawal (pull out) method, but I'm going to buy a plan B pill in the morning anyways. What could it possibly be? Maybe just my normal period? Could sex trigger an earlier period? What should I do?