Where's my period??

Ok. So. I had unprotected sex with my SO last month (Aug 13th) He definitely came inside of me. I immediately went to the bathroom and tried to pee it out.  We had sex last weekend (pulled out) and then after I was bleeding. ( for like a day and a half). We had sex (Sep 4th) and I wasn't sure if he came in me so I took a plan b just to make sure. My period was supposed to come on yesterday but all I've been getting is cramps and light bleeding.  Does this mean I'm pregnant? Or is the plan B throwing off my cycle since I took it the day before my period was supposed to come on? HELP!!!