Swelling of the left side of the face during menstruation may be related to mumps, oral diseases, skin diseases and other reasons. 1. Mumps: Inflammatory lesions in the left parotid gland due to infection, parotid duct obstruction, autoimmune reaction and other factors, resulting in swelling of the left side of the face. 2. Oral diseases: such as gingivitis, which may cause swelling and pain in the gums, and if the infection spreads, it may cause swelling of the face. 3. Skin diseases: such as allergic dermatitis, if the left side of the face is exposed to allergens, it can cause localized allergic inflammation, swelling and itching of the face. It is recommended to go to the hospital in time to clarify the cause of the swelling of the left side of the face during the monthly period, and carry out standardized treatment under the guidance of the doctor, so as to avoid delaying the condition.