What is going on? Help please!

Today, I’ve kind of felt like complete crap honestly. It didn’t start until I got home from school, around 4. I was just doing my homework and everything, and then I had like a weird nauseous feeling and a pain in my stomach, it was weird. I just got off my period so I know it’s not pregnancy, plus I’ve taken two tests, negative. Anyways, when I lay down, I’m fine, it doesn’t hurt, it doesn’t make me sick or anything, when I sit up or walk around, my stomach starts hurting again and I get nauseous and it’s only in one certain place, like at the top of my stomach, I’ve had light heart burn and like, been burping a lot and it tastes AWFUL, almost like acid reflux or some sort. I’m thinking maybe a stomach ulcer but I’m not sure? What do you guys thing?