Hi.. Can understand your concern.. As per your complain a painless sore can be due to an infected and non vital tooth which can cause pus formation near the root tips of infected tooth leading to creation of a drainage opening on the surface of gums at the level of affected tooth.. Other reason can be a viral infection known as herpes. In herpes boils or blisters occurs on the lips i.e. outside the mouth but can occur inside the mouth and is mostly painless.. It can be a usual canker sore and there is no specific reason for it. It can also be due to any foreign material got stuffed into the gums carrying bacteria along with it causing infection and pus discharge.. I would suggest you to see a dentist and get evaluated for the exact cause of blister formation and treatment is based on diagnosis.. If it is due to infected tooth, root canal treatment of tooth followed by antibiotics can be relieving.. In case of canker sore it is self limiting condition and resolves in a week time.. In case of pain numbing gel like oragel can be applied. In case of viral infection antiviral ointments, painkillers and avoiding any intimate contact with others should be done.. In case of gum infection cleaning of the gums by a dentist known as scaling and antibiotics can be helpful.. But any treatment can be taken after the exact cause is known.. Hope your query is solved.. Thanks and regards.. Dr.Honey Nandwani Arora.