I used to have something like this when I was about 7 and intermittently since then. It starts when I am still asleep but lasts for some time after I wake up. Its a rare thing and usually triggered by a nightmare I can't remember. Its a lot like the feeling of being trapped, and of there being insurmountable danger. Its similar to a panic attack but there is a weird pressure feeling. It is difficult to be alone and have one of these because of the intense fear.