Thanks for asking the query,
At the time of surgery , you had a trauma near the tooth which turned yellow and sore.
I suppose that due to food lodgement there is infection at the site leading to pus formation .
Get an x-ray of the tooth done , if it is an abscess
it need to be drained .
Take complete course of antibiotic and analgesic
Use saline water gargles 3-4 times a day, use antiseptic mouthwash gargles like chlorhexidine
twice this will prevent infection and promote healing .
oral ointment topically over the affected area .
Hope this helps out.