Why is my Baby waking up and screaming?! HEARTBREAKING?

My baby does this an hour or so after I put her down for bed each night. My thought is that since I rocked her to sleep and moved her to the crib, she got startled and scared and didn't realize how she got there. Now it is dark and she doesn't know where her mama is. Either that or a nightmare....I've struggled with this because I am trying to teach her to put herself back to sleep but when it is a horrible genuine cry instead of fussing, I do go get her and comfort her. Usually there are big crocodile tears - heartbreaking.