Burning stomach!? please help!?

about mid day yesterday i started getting this burning sensation about midway between my ribs and my belly on in my stomach. thought maybe it was gas or acid reflex or something. i have a Dr's apt later today for an ultrasound and i'm not supposed to be eating or drinking anything until an hr before my apt time. so i can't take anything for it. well last night it started to make me feel very nauseous. like if you have an upset stomach and you get that feeling deep down in your stomach and it stops right at your throat before you puke (sorry if tmi), well that's what it feels like except it stopped right under my ribs! i went to sleep thinking it'll p by morning. i wake up and stand up and it came back! sat down inside the truck to drop my husband off at work and few mins down the road and it seemed to disappear. then when i stood up to come back inside the house it came back!!! pressure on the area makes it worse. i don't like feeling this way at all! what is going on? what could cause me to do this??