A friend of his told me he had a severe fall at home (secondary to leg weakness from low potassium).
Because of the low potassium they were also treating him with IV potassium. He was still very weak though and didn't think he would be able to get up the stairs to his apartment.
Low potassium can cause severe fatigue - I've experienced that. I'm glad this was discovered and Andrew, I'm glad you were given IV potassium while in the hospital - but I think it's very likely you may need to supplement extra potassium every day. I have to take 1200 - 1500 mg. of potassium every day in divided doses - otherwise I experience symptoms of hypokalemia pretty quickly.
I think people with ME/CFS are prone to low potassium because of the nature of our illness.
This post explains how people with low potassium can have "normal" blood levels but low intracellular potassium.
Something simple you could try is to drink several glasses of low-sodium V-8 a day. It's high in potassium and a quick and easy way to get it. Too many of the other foods high in potassium like bananas and sweet potatoes are also very high in sugar. And if the low-sodium V-8 helped you, you could think about taking a potassium supplement daily. Because your levels should not have gotten that low to cause a fall and to require an IV - whatever caused the low potassium which was treated in the hospital I would bet is still ongoing.
Also, low potassium can cause nausea and loss of appetite.
And one other possibility is low phosphorous. I've recently been dealing with that and it too can cause severe fatigue and nausea. One of my symptoms was loss of appetite, especially in the morning - but the fatigue was the worst. It's a long story how I first discovered low phosphorous - had to do with thiamine and refeeding syndrome. Anyways, I have to take phosphorous daily as well as potassium. Basically I treat myself because I don't go to the doctor every time I get extra tired. They wouldn't know what to do.
Something simple to see if low phosphorous is an issue is to drink a lot of milk or kefir and see if symptoms improve.
But overall - since you did show low potassium in the hospital, I think it's very likely you're dealing with it on an ongoing basis and it could explain a lot of your symptoms - severe fatigue, nausea.
Andrew, I'm really sorry it's been so hard for you for so long! This illness is - words are failing me here unless I use pretty bad ones that might not be appropriate!