It might be worth having an X-ray. Yes gas can cause stomach pain but it shouldn't be consistent maybe stay away from fizzy drinks. I found myself after having surgery to be intolerable to fizzy drinks and bread especially burgers I cant eat them. Oh and yes the pain might be telling you that's enough! I know everyone is different for me if I eat to much I get a lot of pain especially in the areas you mentioned. I know you said your is the drinks but they can act as a replacement instead of food especially when its gassy/fizzy it fills the stomach up quick. I mainly stick to water or non fizzy drinks. I make sure to eat first then glass of water sipping slowly afterwards. Also remember fizzy drinks erupts the stomach as it collides with the acid. This is my experience just thought I'd share. Hope it settles for you.
