What does it mean if you are petting your guinea pig on the head and it suddenly jerks its head up?

Actually, when I pet my guinea pig on her head in a bacwards stroke, she kind of jerks her head in an upwards motion. I guess she either sees the darkness and wants you to move it, she thinks she is hiding and she thinks she is making the whole bigger, or she has a feel good sensation in her nerves and has a simple good feeling in her nerves.