Hi.. Thanks for the query.. As per your complain lesion above the tooth that has undergone any dental treatment can be due a few reasons.. It may be due to traumatic injection given to you during treatment.It can cause injury at the injection site leading to lesion formation and pain.. Other reason can be that in case of deeply infected tooth if the filling is done accidentally directly over the tooth nerves and blood vessels present in the central portion of tooth can cause inflammation of pulp and pus formation near the root tips of tooth causing lesion formation and pain.. One reason can be that during treatment any instrument got accidentally injured the gums or the filling material got impacted into the gums causing infection.. Treatment can be decided only after clinical examination by a dentist to rule out the cause for infection. By the time you can take painkiller like Ibuprofen if u have pain and not allergic.. If the cause is due to filling over the nerves removal of filling followed by root canal treatment and antibiotics can resolve the lesion. If infection is ruled out antibiotics can be helpful.. In case of any foreign material impacted in the gums can be treated by scaling... Hope your query is solved.. Thanks and regards..