Sleep paralysis folowed by panic?
Sleep paralysis is where you wake up but your mind forgets to tell your body, so its still paralysed (to stop you acting out your dreams). Can feel like someones sat on your chest (you try and sit up, can't, so the brain asumes there's a weight on top of you). Can also be acompanied by halucinations (your still half in the dream, half in the room).
One of my friends had it once and was convinced there was a ghost straddling her and pining her to the bed. From what I read thats a fairly typical sensation. I tend to go through phases of it, it's liek dreaming, once you know whats happening its a lot less scary and you can lie back and wait for it to pass.