“When should you go to the ER for low potassium?”

Unless you are on a diuretic, there probably is no reason to do so. Leafy greens (spinach), citrus juices, tomatoes are all good natural sources of potassium. There are also supplements and the sports drinks (Gatorade) are also good sources. Diuretics are the most common reason one's potassium gets low. Heart palpitations can be an indicator, as well as extreme fatigue. For a healthy woman of your age who is not on a diuretic, you should be able to get enough potassium orally without having to go to a hospital.

Not something that is typically emergent and you can try to eat foods in high potassium. Bananas/potatoes etc. if you have diarrhea or vomiting then you should go.