Small bubble on the outside of gums below the tooth i had filled 3 years ago. sometimes holds puss, sometimes blood. what is this? how serious is it?

Get it checked: Regardless of what stage of treatment you are at, any level of swelling is not "normal". Whether you are not on antibiotics or not, it would be best to contact your treating dentist for specific professional advise with your condition.

Based on your: Description this is a sinus tract related to a tooth infection that has entered the bone and gum from the adjacent tooth. Please seek an exam w your dentist, an endodontist, or a periodontist to get a proper diagnosis and treatment options. A root canal may be in order or periodontal treatment or both. Don't delay! good luck !

Fistula: Probably a fistula. It is where an abscess tooth drains the pus of the abscess. The tooth needs a root canal or extraction. While it can go a long time without pain, it could suddenly blow up to be a big deal. The nerve of a tooth can die either from a cavity or trauma. This can be without an infection or antibiotics suppressed the initial infection. But sooner or later bacteria find the it.

Dental abscess : Dental infection due to tooth abscess, gum abscess or combination of both. See your dentist for evaluation, x-rays and treatment. You are always presented with a few options. Good luck.

Infected tooth: Probably an abscess with draining fistula. Serious. See Dentist. Referral to Endodontist or Oral Surgeon may be in order.