Low appetite, can't seem to eat much more than half the calories I should be getting?

for the past two weeks, I haven't been able to get down more than 600 calories a day or so. I know anything over 700 makes me feel really sick. I've been in a lot of pain from Fibromyalgia recently, and haven't been able to exercise much, but I know this isn't enough even for sedentary people. I'm not trying to diet or lose weight, but I would hate to gain any. what can be causing this drop in appetite? what'll happen to my body and metabolism if this goes on much longer? I'm having an IBS flair now too, but usually when that happens my appetite increases, since food is going straight through me, but this time I'm so far from hungry it's weird. I haven't started any new medicine in the past month, but could it be medicine related? I take Yasmin and Wellbutrin in the Am and Zinc and Biotion for skin/hair/nails and Tylenol and Neurontin and Promethazine at night, but I have been on these for awhile. I am 24, female, 5'1", about 110lbs, if that helps.