hello there ,
According to your clinical history i suspect your upper molar tooth has supra erupted that is it has gone migration in the space created by lower molar extraction in an attempt to form occlusion.
because of SUPRAERUPTION the occluding cusps of the upper molar might be truamtising the lower gum tissue.
This has lead the lower gum tissue to undergo inflammation and infection in this area may lead to formation of a blister
or an abscess
It needs to be treated by thorough professional deep scaling of gums and root planing of the teeth to remove all the irritants ,this will help in resolving the symptoms followed by a course of antibiotic and analgesics.
meantime ,i would suggest you to go for symptomatic treatment at home like gargling with lukewarm saline water or antiseptic mouthwash like chlorhexidine
i hope this helps ,