Hi.. Can understand your concern.. As per your complain it is true that if a root canal treatment was not done properly it can be initially give no problem but after some years there can be recurrence of infection leading to pain and swelling causing Root canal failure In case if the filling in the root canals is not till full length there can be chances that there can be bacterial growth in the empty space of root canals with short filling causing infection and pain. If the filling is overextended that is beyond the root tips can act as a foreign material for the body and can cause inflammation. Other reason can be broken filling over the crown of the tooth if capping of tooth is not done leading to fracture of filling causing food debris and liquids along with bacteria seeping into the canals leading to infection. Although the chances of facial nerve infection doesn t seem acceptable in your case and it is happening due to re-infection in the tooth and all symptoms are of dental infection only.. I would suggest you to take the advise that whether re-treatment with root canal or extraction can solve the problem. For pain and swelling you can start taking antibiotics like augmentin and painkiller like Ibuprofen along with serratiopeptidases . 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.