Maybe Achalasia. The esophagus interferes with swallowing and gags the person. It could even be a form of Anorexia. See a gastroenterologist.
I'd recommend going to the doctor. If it's just a dull pain below the sternum I wouldn't think you'd have to worry about it being your gallbladder (generally that's a sharp pain), but you could have more going on. You've lost a lot of weight without trying, so I think addressing this with a doctor is best. I had similar pain recently and all it was is acid reflux, but it wasn't constant or to the point that I couldn't eat.