I usually sleep on my back without ever using a pillow (just lay completely flat) and I do experience sleep paralysis sometimes but it was never from that sleeping position - it was from either:
1. falling alseep with one of my arms laying accross my face/forehead (the forearm was resting on my head)
or
2. falling asleep on the couch when my head is propped up and my legs are bent with my knees up and closer to my bod.
Every sleep paralysis incident has resulting from one of those 2 unusual positions for me, and I'm sure it has something to do with the resulting constricted/awkward blood flow conditions that my body is not used to falling asleep under.
And those sleep paralysis conditions were so frightening that I am very careful now about what position I am when I rest on the couch or bed, and haven't had an incident in a long time.
What usually happens is that I sort of semi-consciously wake up due to some some abrupt sound- usually something indicating impending danger. It's either some foot steps of someone quickly approaching me or even one time what sounded like some wild animal clawing around frantically on my bed.
edit: oh yeah one time it was from someone vigorously trying to open my bedroom door (I always lock it as security precaution so no one can sneak up on me
) but I was paralyzed couldn't move! All I wanted to do is desperately reach for my firearm before this intruder broke through the door but I coudln't move a single muscle. Then when I woke up it felt so real and so vivid that I still thought someone was out there even though it was a dead quiet now and I searched my house and couldn't find anything....
The scary part was I felt totally conscious but my whole body was numb even my face. I couldn't even call out or lift a finger, meanwhile there is what seems to be something that wants to harm me RIGHT NEXT to me closing fast!! Ugh... hate to even recall it... But then sort of like Kill Bill style I start being able to wiggle a toe or finger and little by little by body wakes up and I burst up and find no one or nothing there. So it was like I was dreaming. So it's really hard to determine if it really was just a nightmare or if I woke up from a nightmare with it's memories fresh in my head but my body was still sleeping...