Pimple like thing that comes and goes on my gum/swollen cheek, had pulpectomy 5 years ago. what can i do to get stop it from appearing again?

Infection present: When this type of occurrence is present it is a sign that infection has resurfaced. It could be a reoccurrence of the original problem or it could be a new infection process occurring. I would advise that you have the area examined to determine the actual reason for this to be occurring. Your dentist will be able to provide you with an accurate diagnosis and treatment recommendations.

Need RCT: Pulpectomy is the beginning of a root canal. The pimple is most likely a well established abscess that drains periodically and then heals and closes. You should get full root canal therapy(rct) to get rid of abscess for good.

Tooth abscess: It appears that the tooth may have an abscess. The pulpectomy is partial removal of the nerves within the tooth. You will probably need a root canal to save the tooth. See a dentist now for treatment. If you wait too long, the tooth may eventually need to be extracted.