You're being too medically literal. It means I'm teasing you.
If I said I was pissed up on Guinness last night (which I was by the way) it would mean I was drunk.
If I said I just pissed myself reading your last post it would not
mean that I was incontinent.
Although of course I could just be working with a Blackberry and shaking myself off as I type, very slowly, with the other hand.