Hi.. Can understand your concern.. As per your complain a small sore on the superior labial frenulum can be a mucous retention cyst or a small mucocele due to a blocked minor salivary gland which causes spilling of its content into the soft tissues appearing as a lump.. It can also be occurred as a result of trauma or injury caused by some hard food items leading to a minor inflammatory process and appearing as a lump.. A canker sore can be there but there are less evidences of its occurrence over the labial frenulum.. You need not to worry as it will heal itself slowly and there seems to be nothing serious.. Maintain a good oral hygiene and use antimicrobial mouth wash. Hope your query is solved.. If you find the answer helpful please write a positive review and click on find this answer helpful as a token of appreciation.. Thanks and regards.. Dr.Honey Nandwani Arora.