If someone a1c was 5.5 can it turn 6.2 in as little as 3 months if they were eating unhealthy?

Yes, it can: The HgbA1C is a snapshot of the past 3 months of ones blood sugar levels. If you have been eating a high sugar, high carbohydrate diet, one can certainly raise their levels by that amount over the month interval. In reverse, if your level is elevated and you get your diet on control, you can drop it by similar amounts over that 3 month time span....