I had surgery a couple of months ago and couldn't work out for quite awhile.. it sucked! I don't know what all you can/cant do, but maybe try doing punches while sitting or just walking around? Anything to keep moving, no matter how small the movement is, seems like it would help. I walked a lot.
I lowered my calories since I couldn't work out, but I think you just have to listen to your body and do what feels comfortable.
Sorry you hurt your back though!