I'm not a nutritionist or anything, but is there a possibility that you are trying TOO hard to lose this weight? Only taking in 1100 calories seems pretty low - is it too low? Is your body going into survival mode?
The only other thing is to look again at your activities and intake - it takes 3500 calories to gain a pound, so unless your body HAS gone into shut-down, somewhere the equation has to be wrong :-(
There's no other medical reason that I can think of unless you've got an under-active thyroid? But only a doctor can tell you that.