Next time someone starts to say something square up your shoulders, look them straight in the eye and say something like:
WOW - it's amazing that you know and understand more about my situation and my puppy than MY VET and I do. You must be psychic or someting!
Then walk away. You don't owe strangers any details and the more info you provide to "nosey poseys" the more likely you will have to defend your actions. This is YOUR pup and it is YOUR business.
Better yet, next time you go to buy pupy formula, call me so I can tell people to mind their own business

I haven't had the chance to do that lately, I think I am due the pleasure