Can an abscess on the gum come and go?

I few days before Christmas my 8 year old daughter noticed a white in color squishy bubble on her gum above a tooth that had been bothering her, we had took her to the dentist about a week before the bubble appeared for the tooth that was bothering her and the dentist did xrays and said everything looked fine and that the tooth bothering her was most likely hurting because she has a molar coming in behind it. Then a week later that's when the bubble appeared above the tooth that's been bothering her, we had to wait a week to take her to the dentist for the bubble because the dentist was closed for the holidays. 2 days before that appointment the bubble was gone, no signs of it. We took her in anyway to have it checked, the dentist couldn't even see where the bubble was, they did more xrays and again said everything looked fine, they pushed on her tooth and tapped it with the tools and she had no pain at all. So the dentist said he didn't see any problems with her teeth and said the bubble didn't look to be tooth related because her teeth look fine, this was a different dentist from the first time and they both said the same thing. The dentist said if it was an infection that it may have burst and if it was infection it would be back to keep an eye on it. Well it's been about 2 weeks since that appointment and it's back. It looks red this time though, no pain or fever. So I'm stumped. She has another appointment this coming Monday for a cleaning so I'm going to mention it to them once again. Anyone have any thoughts on what it could be? It doesn't look like a blister or canker sore.