Is it kind of a chilly, tingly feeling that goes up and down your spine?
If so, it's possible it's a surge/release of... I forget what the chemical is... I want to say dopamine but I'll have to look it up in a minute to be sure. It's the same kind of feeling some get when that chill runs down their spine taking a late-night superpiss; the feeling they described in The Sixth Sense when regular people are around ghosts.
If anyone knows what I'm talking about, it's actually possible to learn how to cause the release yourself almost like flexing a muscle. It can be eerily soothing when you're stone cold sober and just want to feel good and relax on the couch or in bed or something...
(lol, this feels like it turned into kind of a weird post)
[Edit: Fuck, man, maybe I hallucinated reading about that. I just remember looking into something similar a few years ago when I wanted to figure out what exactly what going on in my body when I inadvertently learned how to, at will, send chills/shivers down my own spine and give myself goosebumps all over. It was pretty weird and I wanted to know wtf I was doing, and remember reading something about being able to trigger a release of some chemical/hormone/protein or something of the sort. And yes, I realize I sound kind of retarded at the moment.]