Why am I so cranky?

I suppose having a stomach ache in the middle of the night didn’t help. I also hate my job (and my management chains’ indecision) so much that my fuse is getting a little too short. A guy I have no respect for said something fairly innocuous, and I snapped. It went something like:

Are you sure you know what you’re doing?

F*** you.

I need to be a lot more civil, but how can you be someone you’re not? My sister is no help. She just yells at me and tells me to grow up. I think I just need to leave my awful job. I wonder if there’s a job where I don’t have to deal with stupid people and get health benefits. I know that’s very unlikely, but without hope I’d just give up and live in a box for the rest of my two or three years (where I’d be without the health insurance).