Please help me

Lexi • 18 and in love with my best friend
Hello:) 
​So I am only 18, and my bf is 20. This past Friday (March 28) me and my bf had sorta unprotected sex. We normally use some type of birth control, but on Friday we weren't thinking straight and I let him go in for a couple of seconds but didn't finish and now I'm really scared that I may be pregnant. He says he hadn't c** at all that day, so if the theory that there's left over sperm if the boys finished that days accurate, then I suppose I'm fine. But when that happened, I had a 9% chance of pregnancy and I don't know.
​Maybe I'm just over reacting, because I do suffer from anxiety. I'm having my regular pms symptoms.
​Do you guys think there's much of a chance of me being pregnant? 
​Btw We've both decided to hold off on sex for awhile.