hello and welcome to HCM forum,
according to the history presented by you it seems that it is a case of re-infection, as root canal treatment
has already been performed on that particular tooth.
the reason for re-infection is improper technique of RCT, also you have not mentioned that whether you had a crown over the tooth or not after the RCT.
i would recommend you to visit your dentist
soon as the infection has spread to the sorrounding tissues as well, but till the time you cannot make a clinic visit, start with antibiotic and anti-inflammatory medication. also perform antibacterial mouth rinse(3-4 times/day).
do not worry and meet your dentist soon.
i wish you good health,