I can't be pregnant right?!

I attempted to have sex with this dude twice in the very beginning of September but he was HUGE, and I guess I am hella small so sex was extremely painful. He would always stop in the middle because I would start crying/yelping. He said he never came and we used a condom but I missed my period at the end of September and now again in October... I'm not on any kind of birth control and I used to be anarexic and struggled with regularly then but now I'm a normal weight and all summer and spring I was regular. I am super confused and pregnancy didn't even cross my mind till just a min ago. I need to calm down right? I can't possibly be pregnant right? I'm 16 and taking 4 collage classes and am varsity cheer captain and all that stress is just making me crazy right?