Twice now, i've had pus-filled spot on gum, which quickly burst causing pain/sensitivity. 3 dentists, 2 x-rays showed no infection. any idea of cause?

Many variables: Did you go to a general dentist or a specialist? If you saw a specialist, did you go to an endodontist or a periodontist? There are many variables. Did the general dentist do a six point periodontal charting around the tooth? A fracture or periodontal abscess may be subliminal and not show up on the radiograph. Was an antibiotic given and did it alleviate any symptoms?

Parulis fistula: is a sign that there is chronic painless infection and your body is draining it out. They do not show up always on an x-ray. Unlike some other types of infection, a chronic gum abscess will not get better on its own. See a periodontist or endodontist for definitive diagnosis and treatment. Good luck.