Hi, I've Been Taking Birth Control Pills For Years. This

hi, i've been taking birth control pills for years. This month, i missed taking the 10th pill on my pack. i was not able to take it but took the 11th pill as scheduled. Since then (2 weeks ago), I've been having brown discharge. i had sex 4 or 5 days after missing my pill. I finished my pack and am due to get my period tomorrow. i'm worried about the chances of getting pregnant though. What is the possibility of this happening? Thanks!