Same thing with channelling.
I've never been possessed but I did inadvertently channel once, when I was asking for help with some magic work and I didn't have any tools with me or means to prepare. I was told what to do and, after, felt entirely drained. So much so I had to go and wash my hands and feet, then sit somewhere where I could plug back into to what I'd done to regain some of the energy still buzzing around. Even then, it took me hours before the energy returned. And one of the effects of the work was, er, a little unexpected for those who had asked for it.
I've also been used in a different way, in the middle of a working with one god, when he departed from what we'd sort of agreed I'd do and decided to add a little sex magic. Protests go unheeded.
Of course, your experience may be nothing to do with magic; I've heard of one person who was possessed by Odhin in a seidr working.
