I'm down too... I'm 5'4", 106lb. Before IBS hit me like a train I weighed 125 at my heaviest. The nausea and D just didn't warrant a healthy appetite. I think the trick is to eat frequently. Have many small meals a day.
How long did you try the calcium? I don't get it because it really should work. Keeping up with vitamins and Ensure is probably doing you a lot of good if you're not eating well. When you can eat try eating really healthy - like fruits, veggies, greens and stuff like liver. Liver is loaded with vitamins.
I have a high motabolism so it is hard for me (sounds like maybe you too). I also just got off the pill so that made me drop even more weight. I'm just trying to eat healthy (but I'm still eating the fatty yummy desserts too!).