If you haven't, check anywhere it could be damp/moist, I do remodeling and find mold in ac vents pretty often, which is bad because it circulates the stuff. Or even around the central unit. Can be tricky to find but sometimes I see it even in the weirdest places like on a tile floor underneath something. Under a sink or inside a window sill that wasn't flashed properly. Even small roof leaks that you don't know about can create black mold on the other side of sheetrock ceilings or walls. Look for slight staining. Black mold can be gnarly, and your description sounds like it could be. Especially if in addition to what you've said it's somewhat recently sprung up and getting progressively worse. I've had repeated exposure and it was aches, pains, fatigue, lower back pain, back stiffness and just a really out of it feeling/miserable mood. When I figured out what it was and took care of it I very rapidly felt 100%. Hopefully you figure out what it is, good luck.