I was dead.
A few months prior to the start of this campaign, my character's family had died in a house fire. My character recalled being hit on the head by a falling timber, being dazed, then dragging himself out of the house. In reality, he had just died from a broken neck, but was spontaneously raised as an undead. In this campaign the reason for undead existing was that a mortal sorceress had recently conquered the underworld and taken the God of Death's position. Not being sufficiently strong to actually do the job of Goddess of Death, some souls slip out of her grasp. I was one of them.
Anyway, I deduced I was undead when I no longer ate, slept, peed, and or bled, which really didn't help my self-esteem much.
I got to be stark raving mad. My mind and brain were slowly becoming more disjoined, and I began to lose control of facial expressions, body movements, etc. Was really fun to physically act out. Nobody else in the party could figure out I was undead, for some reason. They just thought I was nuts. Scared the crap out of my cousin (another PC), who knew my character -before- I had died, when I was still sane.
The party went into a tomb for various reasons (it was the adventure that brought us all together). I was looking for a legendary robe that gave the wearer the effects of a permanent Gentle Repose, so I would stop rotting before other people noticed. Spent the whole time simultaneously pretending to run away from zombies (who had no interest in me, I was one of them) and -actually- running away from the party cleric's turn undead.
Still, nobody noticed. When I later admitted to them that I was, in fact, dead, they were all shocked.
And now we're going to get my soul back!