I had something similar.

Throughout treatments I pretty much held my weight steady, however after the radiation was completed I lost another 15#. At that point I was getting somewhere in the neighborhood of 2000 calories a day (lots of chocolate chip cookies and Ice Cream.... man I miss those days).

After about 9 weeks post rad, the free fall stopped and about a month later my weight started pushing up again. My docs did tell me that the treatments affected metabolism and did not seem to concerned, but did want the weight back up.

Kevin