It could be possible that it's because of teething but I had the same problem with Coco and the vet suggested at 5 months to start brushing his teeth. They have specific toothpastes and toothbrushes that you can buy at any pet store and believe me after I finished brushing his teeth his breath was wayyyy better lol. But ask your vet if it's okay to start now my vet had allowed me to start brushing his at 5 months.
Good luck
