I've messed up!!

Me and my boyfriend have been ttc and I last slept with him on Monday 13th... I was previously seeing someone before I met my current boyfriend and he got back in contact with me last night. I ended up sleeping with him and now I'm in a predicament of whether or not to take the after pill. If I had already conceived what is the effects or damage on a fertilized egg?? I'm so stressed with worry and I know that what I did was out of order but I need to deal with the matter at hand... Do I chance it and not take it? I just don't know. My boyfriend is coming over to mine tonight and I'm going to confess all to him as you doesn't deserve this at all! I fucked up