What should i do? its making me freak out!?

I lived with my sister and brother in law for awhile to get my life straight and to fish high school there..then things took a tole and i moved out..when i was living there my brother in law intrduced me to his best friend kris, who is also married BUT she dont she didnt live with him anymore..one night we were all drinking and joking around and he ended up taking me home with him..we had great but didnt use a condom and i stoped using my birth control not thinking i would meet anybody after my last boyfriend..i havent started my period and i notice my stomach has been getting hard..help me?