What Are The Chances Of Pregnancy When One Is Under Birth Control?

I am currently on birth control but very bad at taking it. Looking at my pack I got it on November 22 and it is now December 20. I still have 4 more active pills. There have been many days I have taken more then one pill a day just to stop menstual bleeding. I last took a pill on Dec 7th to stop bleeding and it did. I had unprotected sex (Withdrawal method repeatedly) on December 16th and 19th. What would you say my chances of pregnany are and when should i test?