The swollen area does not have the normal red color of the right side of her face. Usually swelling of the face there can be due to a respiratory disease such as MG or coryza. In the absence of other symptoms, it could be caused by a peck injury, sting, or tick bite. You could watch her, and make sure that she is not getting picked on, but also getting enough food and water. Sometimes Benadryl can be given orally once at a dosage of 4 mg per pound of weight, in cause of a sting or allergic reaction. If you have Tylosin or have had a past respiratory disease in the flock, that can be helpful.