I am 20 years old, healthy but doc says my potassium is 5.5 and need to be on low k diet. i don't eat a lot of k, kidneys seem fine. what's wrong?

Needs evaluation: The first thing i would do is recheck the potassium- this could be a lab error. You will not have high potassium no matter how much potassium you eat unless you have kidney problems. I advise seeing a nephrologist (kidney specialist) for a thorough evaluation if your potassium is consistently elevated. See https://www.Clinicalkey.Com/topics/nephrology/hyperkalemia.Html.