What type of pain was it? There are so many ways of describing chest pain, to quote Greg House, "each one diagnostically useful". If it was a burning pain, which happens to me, it's probably a bit of acid reflux. The fact that you were laid down, then reached further down makes this a likely possibility. I'm guessing it hurt, you sat up, and it went away?
If it happens again have a drink of milk before you go to bed, or whenever it happens - should be enough to prevent it.
McG