Mouth Blisters Developing: Three Weeks After Treatment

These are common. Are you being too adventurous with your food? There is another kind of blister which is small and often disappears overnight. These are caused by the ducts of minor salivary glands in your mouth and lips getting blocked. Keep a food diary and see if there is a commonality. 
SLS in toothpaste can also irritate and the worst culprit is Duraphat. Try using Oranurse for a few days

You can reduce the amount of Duraphat you use by having a fluoride varnish every three months at your dental hygienist 

HimSteve Yes  they are common especially if you introduce new foods and even now I can occasionally get the odd under, tomatoes are one of my main triggers. 
Hazel x