Animals Living Inside My Bed

Sometimes I wake up and it feels like animals are moving around inside
of my matress.  It's really weird and it really freaks me out and it
happens just about everytime I fall asleep and wake up shortly after.
When I wake up I think "Hmm what woke me up."

Than all this **** is moving around under me and it takes a few
minutes til it calms down.

Why does this happen?

It's not natural.  I'm pretty sure there is nothing living inside my
bed, is this some sort of weird nightmare or something?

Or like I was just going into REM sleep and I woke up and that's it?

I don't know, so if you have any ideas let me know.