Why is she always itching her neck?

I tried to take a good picture but she wouldn’t sit still . So she’s 15 months and she’s ALWAYS scratching in the same spot. It turns red and skin coming off . I don’t know if it’s allergies? If it’s because her spit goes in between her neck and causes this? Because she has dry skin? I really don’t know, and it’s only there ... no other place on her body.