I've said i had a cold before and i got to miss a couple days. I actually did faint at the doctors office which got me out of work aswell, for a day, so you could claim that happened. I highly doubt you'd get fired even if you just didn't turn up because they'd prob just give you a written warning or something. Call in, pretend to have a splitting headache and your stomach hurts, you can do it dude. Just say, this isn't me man, you know, i feel awful, sorry about this but i can't come in today.