I've experienced this many times. I believe that if your not fully paralysed and your aware of your breathing, then when you try to get out, it causes this feeling. Usually I'll let myself go a bit more and then attempt to project again and then there's no pressure around my chest.
I must agree with Clarky on this one...
It happened to me loads, its like your body fighting somehow to fall asleep as if it knew your mind is trying to "trick" it. Also might be a result of maybe adrenaline rising when you think you are getting there and you are getting emotional? When you meditate you focus on your breathing and the trick is to get to a state where you don't feel anything at all, this is what they call the paralysis.
It is not so easy as we might think to cross the bridge from awake to asleep with our mind still awake... keep on trying
