There are a lot of things that can cause that...allergies, pollution, exposure to toxins or mold, systemic
yeast infections or other fungal infections, certain bacterial infections (like Bartonella), Celiac Disease, and some chronic conditions, like Chronic Fatigue/Myalgic Encephalomyelitis, Fibromyalgia, and a variety of autoimmune disorders.
I have it as part of my Lyme Disease and Bartonella so I know how frustrating it is.