I have no idea why you gain weight but I have as well. I seem to be eating more and exercising less. I also noticed that I still crave carbs which I never did before. I used to eat lots of salads and drink water and now it is chips and soda.
I am cutting myself a break because, well, chemo/radiation stinks. I am 1 month post chemo. I promised myself I am going to enjoy Thanksgiving and then it's time to get back to healthy eating and regularly exercising.
My doctor told me my body is still out of sorts due to the treatments and it takes about 3 months for everything to settle down. I hope he is right!