MISSED PILLS! PLEASE HELP!!!

Okay so Saturday I forgot to take my pill (I'm on the combination) so I took two the next day which were the pill I missed and the one I was scheduled to take for that day (this was in week two of the pack). Monday I took my pill as always. Tuesday I fell asleep doing homework, woke up the next day and realized I forgot to take my pill so I took two that day again the one I missed and the one I was scheduled to take for that day (this was Wednesday and also the day of the first pill of week three). I had unprotected sex the following day (Thursday), he pulled out but I'm still worried about pre-cum. What are my chances of becoming pregnant????? And should I take a back up birth control such as the plan b pill or should I just wait???? I'm not sure if taking plan b is safe to take since I'm on birth control that's why I ask if it's a good idea and suggested. Also next week I start the placebo pills.