Can my stomach shrink?

My parents tell me my stomach can shrink. I used to be able to eat a lot of food, but for the past three months I can’t even finish a poptart without feeling like I’m going to throw up. I force myself to eat until I feel like I’m going to get sick, wait like ten minutes, and then start eating until I feel that way again..and I do that for small foods too. (ie poptarts, hotdogs, chicken nuggets, etc.) Any suggestions of what could be wrong with me? Drinks and even water does this to me too.