What are the chances of me being pregnant?

First of all, ignore my avitar. I haven't been on In a year and didnt have time to change it. I am 17 now.

So a month ago, my boyfriend and I hooked up in his car. I got my period that Wednesday, but it was over by Saturday. When we hooked up Saturday night, I was not on it. I gave him oral and he gave me oral. When he ejaculated, I swallowed and none of it got down near my vagina. However, at one point I was sitting on his lap and his penis was against the lips of my vagina. Sunday, after all of this happened, I started my period again. It is now a month later and I am 3 days overdue for my period...do I need to go buy a pregnancy test? What are the chances of me being pregnant? (I am not on birth control and we didn't use a condom for oral or anything)