Hello & welcome,
As you have undergone the treatment of crown placement two years ago, you need to be more careful about your oral hygiene
than other patients.
This is because, crown or bridge placed in oral cavity, tends to accumulate more debris & plaque
as compared to normal tooth structure.
Pus formation in tissue surrounding the tooth indicates presence of Gingival or Periodontal infection & here most likely it's origin is due to lack of optimum cleanliness.
Visit your dentist
, get an x ray
done to rule out underlying caries in the same tooth, get your teeth thoroughly cleaned by dentist.
Antibiotics and Anti-inflammatory drugs will be required to clear the infection. Proper cleaning habits should be exercised by you to keep such infections away.