canker sore turned into blister

I often had canker sores growing up and now get them 2-3 times a year.  They are painful and go away in about 10 days.

I recently had one on the inside of my lower lip that stopped hurting after a week yet turned into a blister.  The blister pops then looks like it's going to heal and turns back into a blister again.  This cycle has been going on for about a month now and I've never experienced this before with any canker sores.  

Any ideas about why this keeps blistering and when it should heal?  I've tried popping it AND leaving it alone and neither seems to make it finally heal and go away.