Hi.. Can understand your concern.. As per your complain pus filled blister over the gums in a filled tooth can be either due to re infection in the tooth below the filling either due to leakage through the filling leading to seepage of food debris and bacteria inside and causing deepening of the infection and leading to inflammation of the nerves present in the centre of the tooth and when the infection extends beyond the root tips there can be accumulation of pus. When there is no escape way for pus it collects in the tissues leading to appearance of a blister over the gums filled with blood and pus. Similar sequence can happen if filling and the tooth is broken and the nerves are opened up. The recurrence of the blister happens as the infection source that is the tooth is still present. I would suggest you to consult a dentist and get an x ray done to get ruled out the cause of swelling and get treated if the tooth can be saved. Root canal treatment can be done in which the entire tooth including the root canals is cleaned, shaped and filled. If cannot be saved extraction can be done. Taking a course of antibiotics can help in reducing infection. 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.