A Foggy Mind and Zero Motivation, How Do I Deal with These Issues?

For the past two months, I’ve been working as if in a fog; it’s hard to concentrate on even basic tasks, even though I used to be passionate about my projects. At first, I thought it was just regular work-related burnout, but now I’m also feeling completely exhausted, physically sluggish, and kind of apathetic. A colleague told me he had exactly the same symptoms due to a hidden inflammatory process in his body and low test results. Has anyone else experienced something similar, and what specific tests should I get to get a complete picture of my health?