Your body is and always has been in calorie burning mode, even during chemo. There is nothing magical about weight loss. Use up more calories than you take in and you will lose weight. Of course there are healthy and sustainable ways of doing that and others that are not. Just be sure you are being consistent and realistic. Working out now and then or underestimating calories consumed just isn't going to work.
I used a an online site several years ago to help with weight loss. They have some excellent tools, lot of teams and message boards, and some great articles and fitness videos. http://www.sparkpeople.com
and it is free.