Wait so she is literally a sleep paralysis demon then.
Sleep paralysis is a real thing, where the person awakes when the body and brain are still in sleep mode. Due to the body being in sleep mode, the person can't move. Breathing is also automatic at this point, so it feels like someone is sitting ontop of your chest. And due to the brain being in sleep mode, it can often cause hallucination. So the hallucinations coupled with the pressure on the chest makes your brain think that there is something, or someone sitting on your chest. So mares, sleep paralysis demons and even alien abductions described in history is sometimes thought to be a result of people entering this state.
