hi and welcome,
Swelling of the gums is the sign of inflammation of the gingival tissue know as gingivitis . This may be caused due poor oral hygiene
, presence of bacterial plaque and calculus inside the gum pockets. Since you have mentioned the swelling is slight and it is in posterior most portion of your mouth i can even suspect Pericoronitis
in your case.
Pericoronitis is the inflammation of gums overlying the wisdom tooth. it often shows infection and formation of abscess. draining abscess and poor oral hygiene can also be related to tonsillitis
This can be confirmed only after clinical examination so i suggest you to visit a dentist
and the evaluation done.
through deep cleaning of the gums and root planing of the teeth will subside th gum swelling and removes the plaque and other debris.
Till you see a dentist. you can do mouth gargling with antiseptic mouth wash like chlorhexidine
and maintain good oral hygiene by brushing and flossing.