Just read the article posted in the link.
As few have commented in the blog, it is most likely "sleep paralysis".
I have that experience many times during my younger days.
Some said it is when certain part of the nerve was slept on that causes the "paralysis" effect. You just need to do a sudden movement and you are out of it.
The older generation will tell you that it is an unknown entity pressing against you.
Well, it's all up to each individual to believe