Posting just for info. 2 years ago, I was frantically searching online for anything about teenage illness involving back pain, extreme fatigue, loss of appetite, for 15-year-old grandson who had just moved in with me following family breakup. It lasted weeks and he was really ill. GP said emotional trauma and potassium deficiency.
He's now recovered and for work-related reasons was tested for hepatitis B the other day. Test showed that he had recovered from hep A in the past: exact same symptoms as above. Fortunately it (apparently) doesn't recur. But worth googling if your teen has one of those worrying kind of malaise episodes that everyone else thinks is psychosomatic but you know it's real. (Apart from symptoms above, urine is very dark.) Said to be rare in UK, but GP was dealing with at least one other 'potassium deficiency' case at the same time, and it's infectious.
Hope this might help someone else.