gomicho, welcome!

I'm stumped too. I was going to say that anything you might be taking, like the melatonin you mentioned, might be causing your problem but you've discussed why you don't think that it is.

This may seem ridiculously obvious at this point but have you thought about what you typically do during a day? Is there an activity where you use your arms or upper back?

I had similar symptoms when I used to knit, for instance. I wasn't knitting intensively either--just a little each day.

Sorry I can't come up with any better answers for you.