Major weight gain on steroids?

I am 22 and have had joint pain for years,i broke out with sores on my leg and dotors think it maybe sarcoidosis. The rhumetologist put me on the steroid prednisone. She said if i did gain weight it would be very little.now i have been on the meds for a month and i am 20 pounds overweight.none of my jeans or bottoms fit. only pj's. I feel so helpless,i was trying to lose weight before swim suit season and didnt make goal but 20 pounds later i cant get myself into one! I have been asked about 6x if iwas pregnant! anyone know the fastest way to lose this weight?I have weaned down from 4 pills to now 1 1/2.but my body is starting to swell from not taking them and my dotor has totally been useless and doesnt have an answer for anything. I want to be healthy, but the weight gain has seriously taking a toll on breathing,and moving in general, i feel like my skin is going to rip...help?