I do this on a regular basis. I've done it many, many times when going to sleep at night, and also when trying to take a nap, or waking from a nap. It's like a rush of adrenaline. It feels really yukky, so I can sympathize with you. It happens to me at least 40% of the time, I would say. Even if I'm not feeling especially anxious, this will often happen. Never used to--not even when I had panic attacks in the past. I have no idea what causes it, but know you aren't alone.