I think I also experienced it several times. Basically, I was seeing what I would see if I was awake, but I could not move and if I tried, I felt I was moving "in my imagination" or in a dream, but not in "reality".
However, I never felt any terror while experiencing this, which makes me doubt it was sleep paralysis. It was just like dreaming I was lying on my bed, unable to move.
It only happened while attempting to nap in the afternoon. Now I try to always listen to music while napping (preventing me from going into deep sleep), so I don't have this kind of experience anymore.