Am I pregnant? Could it be another condition?

Hello,
I'm a 20 year-old student, and recently I've been having upper abdominal pain with scattered bursts of lower abdominal pain. I've been feeling exhausted lately, even though I've been eating well and sleeping for about 8 hours each day. Furthermore, my stomach is bloated and I feel my breasts are heavier. I've also experienced a lot of gas, and usually the pain eases after passing gas.
I'm on a birth control pill and my last period was on October 13th, and I haven't been intimate with anybody since September 25th, but in september I freaked out thinking I might be pregnant. I've taken two home pregnancy tests and they are both negative. Is it possible that I might be pregnant? Can someone be pregnant and not produce hcg? What else could this be?