Once upon a time I could drink a full pot of coffee every morning and feel fine. Now a single caffeinated (sugar-free) soda makes me feel terrible: racing heart

Once upon a time I could drink a full pot of coffee every morning and feel fine. Now a single caffeinated (sugar-free) soda makes me feel terrible: racing heart, anxiety, shakiness. I've tried exposing myself to more caffeine to get used to it again. No dice. Bodies are weird.