Doctor Prescribed Diet + Exercise

I have been having stomach issues due to a viral infection for about 5 months now which has resulted in a lot of food sensitivities that cause severe stomach pain. I recently started seeing a naturopath as my family doctor and gastroenterologist were unable to provide any recommendations on how to alleviate the symptoms. The naturopath has put me on a very strict diet that is making it very difficult for me to get in enough food each day. I'm averaging about 900 calories a day. This diet will only last 1-2 weeks so I'm not really worried about starvation mode or gaining weight back after. This diet is all about improving my health and the weight I lose in the process is just an added bonus. What I am worried about is losing the progress I've been making in the gym (building muscle and improving aerobic fitness). Since my calories are so low, should I be avoiding the gym while on this diet? I don't want to push my body too hard as I'd really like this stomach issue to go away. I've been out of the gym for about 2-3 weeks now and I am definitely starting to miss it. Does anyone have any advice on exercises I can do that won't use up too many of the already really low calories I am getting? Or should i just avoid the gym/exercise until this diet is finished and then start a new lifting/cardio routine once I'm healthy?

Any advice you guys can give me is really appreciated!