Go back to the doctor and complain, complain, complain until they do something. It sounds like Lichen Planus - but something can be done about it.
How big is the blister as it could be a mouth ulcer, there are some good creams you could try. I use Kanolone oral paste, be careful when you apply this though as it sticks to any part of your mouth where it is wet. I usually apply it at night and the pain is almost gone by the morning. Silverfox67 is correct though, if it has been like this for some time, go back to your doctor, he may be surprised to see you still have it.
Zilactin B is sold at most pharmacies and Walmart type stores. It's some ugly brown gel. You dry the spot and apply it to the canker sore with your finger. Be prepared. The pain is horrific but then it dries and keeps it covered for much or all of the day. It costs about $8.00 for a tube that lasts a very long time.
The dentist probably nicked you while working on your teeth. Canker sores can get pretty big and be very painful. They are NOT herpes type cold sores just a virus that was in your mouth and found a home because of the dental work damage.
Thanks, for info. I went to ER and found to be thrush. Not bad yet caught it in time.To many meds, i was taking caused this. Well once again thanks. U have a good day! Take Care!!