I have also had this happen. I went in for pain associated with a past cancer. Anyway at intake it was normal and then about 2 hrs later they tested me again and it was drastically low. They used different bands, monitors, etc. And said they would have to admit me if it didn't go up before my final paperwork got to me. They kept me forever. The cause? Not a d@mn clue. "Just one of those things, follow up with your PCP". so I definitely relate..the potassium thing is weird though, did you go to your primary care physician for follow up?