What Causes Increased Bowel Movements And Fullness In Stomach After Unprotected Sex?

Hi, I'm 18 and I had sex on Jan. 13th, he did not cum in me, but we did not use a condom. We have done it many times without one too. However a few days later, like on the 16th I started having stomach aces, and was up during the night pooping and hurting and I felt full, like I ate too much. The next few day up until now, I've been still having werid feelings in my stomach and I poop a lot, and sometime I feel like I'm going to start my period, but I haven't yet, my period should come sometime soon, but I'm scared that it might not come. Last month it came on Dec. 18th, and it's Jan. 18th today and I haven't started and I just feel sick to my stomach. Could I be pregnant, have gas, or what?? I'm scared. My period should be here by now. But could I just be stressing? I just don't know.... Help me please.