In my experience, the first things to look at are toothpaste, and
hard candies.

There are papers on SLS and triclosan causing mouth irritation. (SLS is sodium lauryl sulfate). Some of the fancy new toothpastes are very irritating to people.

Sucking on breath mints, hard candies is the next thing to watch for. Chewing gum, etc.

Next up are vitamin deficiencies:
Low niacin is called Pellegra ...and mouth burning/pain and swollen tongue are
signs of this.
And low zinc status, may be contributing.

Mouth pain can occur with some food allergies...esp shrimp and shellfish.

Low Biotin too. Poor Vit C intake? Low CoQ-10 (from taking a statin?).
So trying at B50 and Vit C might show improvement.

Next come drug side effects.... using asthma inhalers? Not rinsing the mouth after a steroid one.
Taking an ACE inhibitor?

Some viral illnesses cause mouth/throat/tongue pain... shingles, and Mono are examples.

I won't repeat the other posts here, on autoimmune causes, as that seems to have been well covered.
Sometimes one has to be a detective of sorts, to ferret out puzzling symptoms.