Why does my stomach feel like it's full of acid?

My stomach usually feels like a burning pit of fire, like I swallowed an entire bathtub of acid. Sometimes it comes up a bit as throw up into my throat. The other day I tried to eat, and it's seems like my stomach is so full of acid that the food just floats on top of it. I opened my mouth and vomit (previously the food) slid up my throat and I puked right into a sink. The other day I finally vomited up a bunch of food and later tons and tons of pink acid. My stomach didn't feel bad after the acid was gone. But now that I've eaten again, my stomach feels disgustingly acidic again and burns very badly. It really hurts and I constantly feel on the verge of throwing up. I am really tired of feeling sick.