Hi Dear, Welcome to HCM. Understanding your concern. Thanks for your query .As you mention that you have lip pain as you star wearing mouth guard .Well it could be possible that your mouth guard has manufacturing defect like some sharp point or bulge on that area which continuously irritating mucosa and creating pain sensation. It could also be possible that there is a canker sore on your lip due to continue irritation or any fungal infection because of mouth guard. I would suggest you to stop using mouth guard and consult your dentist ,so that he/she can properly examine mouth guard and your oral cavity and get your mouth guard corrected if there is any defect.it is very important to get treatment ,because continue irritation can also cause cancerous lesion(rare). For now what you can do is rinse your mouth with saline warm water 2-3 times a day,apply equal mixture of hydrogen peroxide and water and take over the counter medicine like ibuprofen and paracetamole for pain and swelling. Hope your concern has been resolved. Get Well Soon. Best Wishes, Dr. Harry Maheshwari