im still young, need advice and help! D:;;;

Well, my Boyfriend and I had sex about 3 weeks ago, on March 31. And again on April 1. We used a condom, but it did not break. I wasn't sure if it slipped off or not, but my boyfriend said it didn't and I trust him. It didn't have any cuts or anything. It was on the whole time. And i got my period on April 9, and i should be off it tomorrow, April 16. But I've been worried if i'm pregnant or not. I'm SUPER worried, because my stomachs been feeling queezy and nauseous. I've gotten headaches too, but that maybe from stress in school and lack of productivity. But I'm not sure, and I'm super worried. I'm still young and just thinking this can't be true. I canNOT be pregnant. But my period came on time, as expected. but i'm still worried. I've also done a few exercises, like sit-ups, and i don't know if that's the one affecting. I keep rubbing my bellie and feeling something odd, but it maybe the intestines or stomach. I"M SUPER SCARED. PLEASE SOMEONE TELL ME SOMETHING.

(Wrote that couple days ago and no one seemed to reply)

And i'm adding to it.
It seems like my stomach, (located above the belly button, right??) is out. O_O
Any suggestions? I feel gassy and want to burp a lot, and it feels nauseous. Not very confortable. And its a bit hard when i poke it or put pressure on it. Could it be acid reflex?? 
PLEASE REPLY QUICK> !