Yeah it fits in with many accounts I've read about.

I've never seen anyone whilst going through the transition to a Lucid myself but have felt a dark presence a few times, I usually just hear whispering, occasionally screaming in my ears and feel like I'm lifting up into the air end over end, once I felt like I'd fell through my bed and something grabbed my ankle, pulling me across the floor.

There are accounts dating back to Medieval times about the Old Hag which sounds pretty similar to your experience, a dark figure who would sit on people's chests and hold them down.

A lot of people on here say you only get SP if you have a medical condition but I'd never had SP once before I started LDing, since I started a few years ago though I've had pretty classic symptoms of it many times!