Maybe don't do or say anything right now and just see if she brings it up again. Then if she does, y'all can talk about it. Sounds like it was just a spontaneous moment of sleepy closeness, but if she swings both ways and had stronger feeling about it than you did, it has the potential to throw the dynamics of your close friendship off if she wants things to progress further. Romantic feelings do have a way of throwing a monkey wrench into good friendships, don't they? This happened with a guy friend of mine, and things have never been the same between us. (He felt romantic and I didn't.)

You're in that phase right now where it sounds like you're a little freaked and want to understand what happened and categorize it and maybe file it into some proper category so you can feel like you've dealt with and finished with it. That's why I think you need to take a day or two to sit with it and just fully process it and not rush in there and say something that would potentially hurt her feelings or damage your friendship. Does that make sense?