it could be sleep paralysis. the feeling of something pushing down on you could just be the inability to move. in my experiences when i used to have sleep paralysis a lot uncontrollably it felt like there was a scary presence in the room. i looked up the wiki thing on SP and it talks of audio and visual hallucinations and an evil presence in the room. i have definitely had audio hallucinations no visual ones though. but this could explain what youve been experiencing

Sleep paralysis - Wikipedia, the free encyclopedia