Getting back to Exercising

I had RNY surgery 4 months ago and have lost close to 80 lbs. I was walking on the treadmill but that started to slack because I would get a stabbing pain in my left side. Now I find whenever I do any type of exercise I am totally wiped out to the point where Im really nauseated. I also noticed that my leg muscles feel really week. Could this be because Im losing weight too fast and need to get back to walking to strengthen my muscles? Can someone offer any suggestions to a good exercise plan.  Thank you

Hi: I find that I can not exercise if I have not eaten at least 2 hours prior.  I usually come home from work, eat dinner, digest for about an hour, then work out.  Maybe you need to eat something an hour or two prior to working out? Hope this helps!!