I've had that exact same problem for almost 40 years! The first time was when I was 19, napping, and
woke up unable to
move, feeling like someone or something was pressing me into
the bed, like a heavy weight on my chest. I kept trying to
scream, but only a tiny squeak would come out; finally, the episode ended when my leg kicked out and
knocked the cat off the bed, and
I could move again. I was convinced it
was a ghost, if you can imagine that!