It's not listed as one of the side effects of Metformin however it is something associated with high blood sugar
The link between type 2 diabetes and oral health
Diabetes affects your body’s ability to utilize glucose, or blood sugar, for energy. Diabetes can cause many complications. These include nerve damage, heart disease, stroke, kidney disease, and even blindness. Another common health complication is gum disease and other oral health problems
https://www.healthline.com/health/type-2-diabetes/oral-health
There are of course other reasons for oral problems, not everything is diabetes related. Perhaps a visit to the Dr is called for to get it checked out