one of my theories is if work is stressful i lose weight - nervous energy, less time to eat. But it sounds like your stressful work periods involve lots of crappy food!
i am afraid that is indeed the case. My stressful work periods involve meetings from 8am to 8pm with barely any time for eating anything proper, let alone getting any exercise in - the cafe food here is crap and I tend to rely on junk food to get me through (there is a chocolate machine just 50ft from my office. Unless there are restaurant meetings involved in which case I have no willpower and just enjoy the food too much!