Question: Plan B/ pregnancy scare

This morning at about 2 AM, I had unprotected sex with my boyfriend. Unfortunately he came inside me instead of pulling out (I guess he was too carried away).  We immediately drove to the pharmacy and I got option two 2 emergency contraceptive (pretty much same as Plan B) and took it immediately. I have been so nervous because this is the first time this has happened. I am expecting my period in four days. I need help understanding how this contraceptive is going to affect me since I have my period in four days. Am I still going to get my period? Or is it going to mess up my cycle? And is Plan B really effective because honestly I'm still having a pregnancy scare. Thanks.