Hallucinations of entities, demons or other strange presences are common during waking sleep paralysis. I also heard somewhere that the heart pounding thing is a hallucination and your heart isn't really beating as hard as you think it is, although I'm not sure if that's true or false.

This could even be false awakenings as someone above suggested. Either way, truselt me, you don't want to stop having lucid dreams, you want to learn to make them pleasant experiences - because for most, pleasant is a vast understatement.