I get this fear at night. I think it comes from my mother betraying me the way that she did.
When I was younger, I had an intense fear of monsters hiding under the bed, around the corner, in the closet, etc.. It would strike me even while I was at school. Since real monsters were introduced into my life, my fear of fake ones began to resurface. If she could lie to me my entire life about loving me and protecting me, why wouldn't she have lied about monsters hiding under the bed?
I mean what does she know?