"All in your head" means that it's entirely a delusion, an illusion, or a hallucination. If something is all in your head, then there's no component of it in the real world.
But it's a very informal term. Best to say exactly what you mean instead.
I get your defs in other domains like the delusion of flat earthers, or the illusion of a stage magician, or a hallucination of whatever variety. I just don't get the use of the term within the context of physical symptoms or conditions or disease. What does it mean in this specific context?