I've been given the "We found nothing so you're almost perfectly fine so have a nice life!" speech before.
Well what do you want them to do? They can't find anything wrong, they can either keep testing you and never leave you alone, or they can admit defeat and leave you to take care of yourself.
They can't cure or treat something they can't identify. Seriously, I hate it when people get mad at medical professionals because they didn't get their magic instant cure.
I heard a doctor say this online:
"You can get your instant movies and instant food and call someone whenever you want, but you can't make the human body heal any faster than it did 100 years ago."
Not his exact words, but the point is made. The human body and mind are very complex. Alot of times things get past us. There's really nothing you can do about it.
If later on they managed to find something, then you have no reason to complain because at least they found it. If they still haven't, then all I can say is that it's like trying to find something small thrown into a huge haystack, but no one ever tells you WHAT you're looking for and there's a bunch of other shit that could be it but isn't really the perfect fit. Medicine is hard.