I was very recently (actually until the last few days) having the same problem: I’d eat a bit and then feel full right away and do a lot of belching and I’d have upper left abdominal pain just under the rib cage sometimes extending to the side and back. After googling around a bit I got the idea that maybe it’s a hiatal hernia and started doing some excercises (stretching and hanging – google for hiatal hernia excercises) that seemed to have helped. Today I just got back from an appointment with a Naturopath who specializes in gastro issues and after he did some checking around my abdominal area he concurred that hiatal hernia is highly likely and he did some soft tissue adjustments that are supposed to help with that. But even prior to that I was able to eat a more in the last couple of days than I was able to in a while.
So I’d recommend googling for hiatal hernia and figuring out of that fits what you’re experiencing.