I don't know if this is what happened to you, but only after I was off all prescription meds and my body was healing and the best it had been since my 3 years of colitis did I gain some of the weight back that I had initially lost. I never gain on prednisone, but like I said when I was off meds I guess my body was the healthiest it had been and my gut was absorbing the food a whole lot better. So maybe this is a postive blessing in disguise.
If your really concerned about the weight my suggestion to you is to eat tons of veggies. Good for you, fill you up and not many calories at all.