Pregnancy scared!

I started birth control last month on the last day of my period. I then had sex the following day and he pulled out, but after he pulled out he waited like 20 minutes and he went back in. I got freaked out and told him to stop. I have taken all my pills for this month and should start my period in three days but I am scared that there is the possibility I am pregnant and I need some more knowledge!
Any advice or anything is much appreciated!