Have they checked for Lyme disease? 'cuz that sounds a lot like Lyme disease. It's not a death sentence. It isn't even a life prison term. It is, however, a vast panorama of treatment approaches with a probability fan from moderate success to abject failure. Which is true for many chronic illnesses, not just Lyme, so be persistent. Eventually they'll figure out what's wrong, and that's the first step to recovery. I'm sorry your parents aren't reacting well. Part of that is being a teenager. They'll feel guilty later.
They'll test for lyme desise I persume - it's very common in Sweden. However - arthritis is in in the family on my mothers side - my grandma had a lot of the symtoms I'm having now as a child and my grandmas father had it SUPER bad. Also the ER doctor said that even if I wasn't in pain my joints were overly flexible to to the point were they'd want to give help/invesigate based on that alone.