Is It Safe To Take Previfem After Having Unsafe Sex?

Hi! I m kind of having a mini freak out. I take the birth control Previfem daily and have since April 2014. I m pretty good at taking it the same time every day. I am on my 14th day of pills this cycle and there has only been twice that I took the pill a few hours later than I normally do. It has been about 15 hours since my boyfriend and I had sex and he ejaculated inside me. We had talked before the sex about it and convinced ourselves to do it. Now we are kind of in a panic. Should I take plan B? What are the chances of me getting pregnant?