tbraz --- well said!!! I too have always been really thin. Any bout with flu and I would drop 5-10 pounds in a heartbeat on top of being too thin already. After having my ileo for so long, I finally starting gaining weight after recovering from a "bug" in Oct of 2006. I have gained 25 pounds since that illness. I have changed my diet some tho so that might have helped as well. I eliminated carbonated drinks, eliminated really spicy foods (which I love), eliminated almost all caffeine (except chocolate!) and initially really watched my sugar intake tho if I wanted something I ate it. I did have a gastric ulcer and now that is also healed. And I eat lots of carbs - breads, potatoes, pasta. And I try to eat my meals slowly, yea I confess - usually while watching tv for my supper meal. I don't get that full feeling quite as quickly that way. And one more thing, even tho we are suppose to drink, drink, drink --- depending on what I am eating, I may not drink as much while I am eating to keep from getting that full feeling so quickly. BUT, I say with a lot of emphasis, it depends on what I am eating as to how much I drink WITH my meal!!!!
I don't know if this change in my diet was the reason I gained weight finally but I do know that I went many, many years bouncing from a Size 2 to barely a Size 4. And now I am a Size 6-8. I am happy with that, but if I gain more, I would be happy with that as well. BTW I am 5'5" with long legs so I really looked underweight before I gained a few pounds.
Good luck in your weight gain. I feel your pain! I always hated it when I was called "Skinny". Thin was a little more acceptable.