Hot Water in the “Wild”

I’m in school now and figuring out a system for tea has been hard… i’ve committed to cold brews and “grandpa style” in an insulated bottle. i usually refill on water around and about

i find that a lot of times i get hot water from like starbucks or somewhere…. and i just feel this acidity in the back of my throat. is that just a reaction to old leaves or maybe a reaction to hard water buildup on their tap…. oif. i think i know the answer, has anyone else experienced this