Wow, despite my diagnosis of Fibromyalgia, I think Piriformis syndrome sounds exactly like what I have... You don't get Fibromyalgia on one side, even though when I say this, it's ignored. They just don't know what's wrong with me.
I've had every test in the world, just as you, Indi. and it's all clear. And cycling is definitely making it worse. I've had fittings, new bike (s), PT, but it's still here.
The PT did the most help, but I was released in July. Looks like i might have to go back.