Originally created as a discrete warning code for women to explain why they are cranky or unreasonably upset. It describes the bloated and uncomfortable feeling that a woman has when she is having her period. It has also become a phrase that tacked on to the end of a self absorbed rant to indicate that you know you are being stupid and silly, but that you feel like hell nonetheless.
"My life is going nowhere, I am so unmotivated to figure out what to do about it. There is nothing I want to eat in the house. And I'm fat."