My housemate's mood plummets when he needs to eat - he gets irritable, off-balance, easily upset by things he'd ordinarily ignore. He usually doesn't feel hungry, though - often he doesn't want to think about food at all, so it's hard to recognize the fact that he needs to eat whether he feels hungry or not. Once he does eat, his mood returns to its usual cheerful state.

Maybe it would help if you started keeping some fruit, crackers, etc. handy so you could have a small healthy snack when you find yourself feeling blue. Of course, your mood-swings might have a hormonal component as well - if they generally happen at a certain time of your cycle, you could talk to your doctor about it and see what can be done.