There's one theory I learned about that might explain some of these experiences (especially the ones in which people were experiencing pressure on the chest.) Apparently, the brain regulates the body during sleep by essentially paralyzing it. So, sometimes, the mind wakes up while the brain still believes that the body is asleep, which results in a sense of "being trapped." Because of the paralysis, people who experience this describe the feeling of someone sitting on them...actually some researchers believe that these occurences might explain some alien abduction stories. It doesn't match your OBE, but rajking's story seemed pretty similar to what I've heard.