Hba1c value 6. 5% Sugar blood work?

I am 37 years old. I do physical health check every year and my hba1c was always below 5.5%(normal range). I have chronic neck and jaw pain which I have for past 3 years. Seen doctors and specialists but couldn't find the diagnosis. They put me on different medicines which caused weight gain. I gained lots of weight after those pills. I recently did blood test and my hba1c was high. 6.5 Dr told me to change the diet. I tried that for 3 months by making some changes to diet. Did another test now and it's the same 6.5%. So he put me on Metformin tablets. Is this blood sugar value very high? I don't know how I got the sudden high value. I don't eat much sweets and sugar. Will this value goes down with metformin? How often do we need to check the blood sugar blood work?