The bump of pus in my gums are gone after i switched on the aircon (hot weather here) and using mouthwash. what could have caused the pus bump?

Infection: You probably have an infection associated with a tooth. See your dentist to make sure.

Unrelated: The pus swelling is evidence of a dental infection unrelated to the coincidence of turnig on the a/c. You will need a dental exam to determine the best way to treat the infection.

Infection: Most likely cause of a "bump of pus" in your gums is a tooth or gum infection. Turning on the air conditioning and using mouthwash probably just burst the bump and drained the pus, but did nothing to resolve the infection. The bump will return. Go see a dentist and find out where the infection is and get it treated.