For telling 'fibs',thats why,or at least thats what I was told when I was a very little girl,many moons ago.

But, no, seriously,I was reading an article in a magazine recently that mentioned painful mouths,and the suggestion was that the answer lies in the toothpaste you use.There is a possibility that it contains an ingredient that you could be allergic to.
I'll p.m. with the details.
Wish you well
annemari