I personally would get checked out, and I personally think it could be either your gall bladder, liver, or maybe an ulcer. But, then again, I'm no doctor. Hope it turns out okay for you and it's just gas or something minor. Good luck.
I'd definately get your liver checked out.
Well, it certainly could be gallbladder. Have you lost a lot of weight recently? I've been told that doing so will often cause gallstones to form. If you see a gastroenterologist they'll likely want you to have an abdominal ultrasound and a HIDA scan.
Good luck.