Hi Carolynn,
I can't tell you what is wrong, but I can tell you what we did when my husband had this problem. My husband also lost a lot of weight in a short amount of time. Although he was eating he was eating much smaller portions. After a 40 pound weight loss his oncologist put him on Megace, which has helped the problem, it stimulates your appetite. He hasn't gained his weight back, but he hasn't lost any more weight either.
Hope this helps.
Candy