I met women who lost a heap and women who put on a heap. Everyone's metabalism deals with chemo in a different way. I was slightly overweight when I started and stacked on 9kgs, the fluid retention was half of that and the rest is inactivaty from taxotere I struggled to walk out the front yard. I ate super healthy, drank plenty of fluids but it didnt matter. Its now almost 5 months since I finished chemo (and almost one month since all treatment finished, yae me!) and have lost the 9 kilos I put on. It wasnt easy. I'm on Tamoxifen and was really worried about weight gain too but its proven to be a-ok. Eat healthy, excercise daily (when you can) thats all you can do. Things will return to normal (a different kind of normal) but they will. I wouldnt be too concerned about weight gain during chemo it is perfectly normal and is only temporary. After chemo, with a little effort and without the billion drugs you have to take during chemo your body will slowly return to its new normal and everything will improve.
All the very best in your journey.
Big hugs :)