What would cause my potassium to be low with cardiac issues and flank pain? Doc said urine looked good. I'm 24/eat well/do not overly lose fluids

Urinary losses. Unless you have long standing diarrhea or take water pills, you abnormally lose potassium in urine. Need to see a nephrologist. It is possible that your adrenal gland secretes more of aldosterone hormone than normal. Lab tests would confirm this. There are other possibilities too that need to be ruled out by your doc. It may end up that you will need to take a potassium pill or two, long term.