My guess, and without pictures thats all i can do, is that she is itchy and rubbing somewhere. In my experience, arabs are sensitive skinned horses. If shes itchy, she will find a way to scratch it. Below the chin is a pretty common itching spot. Do you ride her with a curb chain? If so, is it to tight? Either way, if a horse starts with the itchies, we usually break out the MSM and give a scoop once a day. It helps with skin allergies and other issues. In the mean time, keep em clean, keep em salved and unless its bleeding or infected, let it be. You can make it worse by over cleaning dry skin.