Check out the pic below. This patient has loads of tension and aggravation in her life:
The great thing about cupping and gua sha is that it is like a roadmap of dysfunction and pain in the body. This happens all the time: I will cup a person's entire back, and there will be one small spot that turns very red or purple. (This color is called "sha".) This one spot, inevitably, is the area that has pain.
This is an acupuncturist's picture of what stress looks like. You may notice that the darkest coloration in this pic is at the very top of the back. This is the Lung area. This patient is also in the process of recovering from some sort of virus. She still has some residual phlegm, so it makes sense that this area would be the darkest.
It isn't just cupping that can give hints as to what is going on in the body. Qi, that energy that comes to the needle and makes the treatment effective, tends to be felt the most in the points that are geared toward the issue at hand. If someone has a queasy stomach, they will feel the qi sensation most powerfully in the points that calm nausea...even if these points aren't located anywhere near the site of the problem. Isn't the body amazing?