Is Taking Plan B Safe After Having Broken Condom During Sex?

My boyfriend and I were having sex with a condom, he pulled out and came in the condom. Then about two minutes later he put another condom on and we continued. About 10 minutes later he was trying to enter but we both heard the condom break and he pulled out. He was not fully inside of me, only the head was. I am now worried, and not sure if I should get Plan B. I had my period on Dec. 1 and ended Dec 7 so I don't know if I am fertile or not.