white bubble on gum

i just got a cavaty drilled and filled , it was a deep one, i experienced severe pain but now that its gone i have this bubble under the tooth that got fixed, which is now turning white, what is it, and what is the danger?

[Jerome Tsang, DDS]

When was the tooth worked on?  

It may be an abscess forming underneath the tooth and its starting to pop out causing a blister/bubble on the gumline area where the bottom of the tooth root is generally.  The severe pain is probably indicative of a problem with the pulp/nerve of the tooth.  When the pain stopped, it probably stopped because the nerve died from the infection and now its spread down into the jaw.  

Time to go back to the dentist.