Sorry to be cliche, but have you tried changing your diet and exercising more? Letting them slip always ends up with me feeling like that. To be fair, the one time it ended up being cancer, but even then the cancer felt better when I was moving around and eating more vegetables. My checklist when I start feeling down: Water, Magnesium, vitamin D3, Fiber, squats, full nights sleep, no alchohol, wake up early every day, have sex, pet a dog, go into the woods.

It doesn't always work, but it's still good place to start. I hope you figure it out.