SLEEP PARALYSIS

often accompanied by slight hallucinations and auditory hallucinations (small voices, knocks, footsteps, etc)

The reason for hallucinating while you are asleep and feeling weight on your chest like your dead grandmother is sitting on you...
It's called sleep paralysis.
It happens when you're under a lot of stress.
It is VERY common in Japan (#1 stress country go figure) and they call it kanishibari
In old times they say kanishibari happens because spirits are trying to take control of your body or some shit like that. So what spirits do is sit on top of you to stop you from breathing so they can take over you.
Many describe the 'aura' as evil or unpleasent.

It happens very rarely..
I doubt you will face it again unless you are under a lot of stress.
Nothing religous happened, but religous people believe monsters are trying to possess them.
And many artists drew paintings of kanishibari.

http://en.wikipedia.org/wiki/Sleep_paralysis