The body has a very intriguing safety mechanism when it comes to sleeping. If you get in a position that you cut off the circulation to a limb, in this case, your arm, the brain receives garbled signals, which you could imagine looking like black and white static on a televsion. If you can’t already, you will eventually feel these signals as pins and needles. When the messages are garbled, it wakes you up to prevent the loss of that limb. So you wake up and have to lift your arm (which can be kinda freaky cos it feels like someone else’s) to move it, but this is not sleep paralysis. Like they said, you’ll know it when it happens. 
Read up on my experienes here: [I am an "old hag" veteran)