you have used the right terminology. it is probably a cyst due to a long term infection associated with that tooth. what I would suggest is immediately see a dentist. you would most likely require a surgical removal of cyst if it is formed, if its just a pus related swelling it can be drained. the tooth if has sufficient bone support can be saved with an intensive root canal therapy multiple sitting. but donot ignore the painless swelling. it would damage the adjacent structures and bone around.