Why do you feel a weight on top of you when you have sleep paralysis?!

It happened to me twice already. The first time was when I was sleeping sideways, the second time when I was sleeping on my stomach. The first time I felt a weight on me that started moving up until it reached my should/side. The second also moved up from my butt and stopped on my waist. Obviously, I couldn't move, I could blink, move my fingers and open my mouth but I couldn't move my body. But why is there the weight feeling right on top?