hi and welcome
What type of diabetes do you have? And what medication are you on?
One of those commercial lattes would probably raise my (non medicated) blood glucose around 1.5mmol/l for around an hour and a half, but be down again at 2 hours. The sugars in milk always seem to affect me more than other carbs of the same amount.
I am wondering whether your blood glucose was dropping before you had the latte? A brisk walk to the coffeeshop? Getting peckish? A flight of stairs or 3?