My girlfriend humps my legs? iin?

When my girlfriend and I sleep together, she usually likes it when I put my leg between her legs (kinda how some people like to sleep with pillows between their legs). I don't mind it, but sometimes in her sleep she'll sorta hump my leg. It's nothing major, she just moves up and down. I like when she does it, this might sound weird but I think it's really cute. Is it normal for her to do this??