Daughter keeps scratching her back and whining about it, but there is nothing there?

[Bella and Hannah's Daddy]

My daughter is two, and keeps telling me, "My back is hurt" i said, what happened and then she will do this weird scratching thing and say, see hurt? And i will look at it and there is nothing there. I don't think she understands itch and hurt. She just woke up and came in crying about her back...i don't get it. I am almost sure that her back itches but there is no redness besides that she keeps scratching her back.

Any ideas?