I had low potassium last year and had to stay in hospital overnight on a drip. Was a nightmare. Not sure why but I was losing potassium through urine and my electrolytes were off as well. I was given this nasty tasting stuff to drink, went back for more blood tests a week later and levels were normal. I think you and I are around the same age, Lindsay.
Sometimes these things happen. My mother is also low on one of the B vitamins, and has to take a supplement. It's just one of those things, no huge deal. Most of us aren't getting enough of one thing or other. Could be diet, could be lifestyle, could be genetics.
And please give the zoloft a try