I have never fasted before, but I have read about fasting to prepare and I am certain that you lose muscle mass when you fast. In fact you lose a lot of muscle mass. Same is true with dieting. I found this set of articles helpfull:
A lot of times people experience problems when they switch their diet in a big way. You definetly made a big change when you switched from eating nothing to eating something. Your body may need some time to get used to it.