That's exactly how it started for me!
I can't give any advice at all, as all tests that I had done at the time came back negative. 2 years later I am still wondering what the hell is wrong with me... It got worse for 3-4 months, I was out of work for 3 months, then I was put on meloxicam for inflammation, propranolol for rapid heartbeat, amitryptiline for pain/anxiety/insomnia. Amitryptiline did nothing apart from turn me into a zombie, so I stopped that pretty soon. I weaned myself off the others in 9 months. I did get better, but the progress was painfully slow. Now, 2 years on, I am pain-free 90% of the time, with occasional flareups, but not frequently and they don't last long.
As for tests: I had ECG five times, standard blood tests, blood tests for bone functions, heart CT, lung CT twice, chest X-rays twice etc etc. What I did not get tested for: viral infections, vitamin deficiencies, allergies. I too had a mystery illness weeks before costo started. About 9 months into costo I was offered tests for a few viruses but by that time I was sick of more tests and no results, and even if I had been positive for a virus, the treatment (take antiinflammatory and keep fingers crossed) would not have changed, so I didn't have them.
What I have in mind: cocksakie viruses (A and B), hepatitis viruses, Eppstein Barr and such like.
I was recently tested for vitamin D, and it came back as very low. Vit D deficiency can cause issues with bones and joints, as well as weak immune system. I am on Vit D at the mo, seems like an easy fix. There is evidence that food allergies (gluten, egg etc) can cause joint pain - but cleaning up my diet did not help me specifically.
Good luck and let us know how you get on.
If there is one advice I would give my 2-year-ago self: do not panic and take it one day at a time. Life with costo is still life and we should strive to find enjoyment in it.