Hi Dear , Welcome to HCM. Understanding your concern. As per your query you have symptoms of pus filled blister on lip which are pointing towards to condition an allergic reaction or infection. The allergic reaction can occur due to various reasons like food allergy, allergy from toothpaste or other dental product or excessive exposure to sun . Infection can be bacterial or viral. Need not to worry. I would suggest you to take over the counter antihistamine drug like benadryl. You should keep your lip moist by applying lip balm containing broad spectrum sunscreen stop using any product on face or inside mouth if you start it recently and apply clove oil at night . If condition still persist then consult dermatologist for proper examination and to rule out any other underlying disease. Doctor may order blood test and prescribe topical/oral antibiotics/antiviral depending upon the condition and topical steroid. Hope your concern has been resolved. Get Well Soon. Best Wishes, Dr. Harry Maheshwari