Thanks for writing in healthcare magic forum,
Your root canal
treated tooth has re-infected ,resulting in abscess
formation under the tooth.
The swelling had become hard due to the antibiotics administered without draining the pus,resulting in antibioma.
Please do visit a dentist
soon and get the pus drained and curetted.
If required,re-rct has to be done.
Maintain oral hygiene
Get dental check-up in every 6 months.
Please get thorough scaling done.