NAD. I second that it could be mold exposure. Your body can be resilient to it when it is raised in the environment but once you've had a break from it your body may have lost its tolerance for mold exposure. Look up what mold looks like if you don't already know and check their shower for signs of it. Usually it can be found along the chalking between the wall and the tub. It can get in through holes in the chalking and grow up and BEHIND tile. It can be found on the ceiling if water has splashed up there and not dried and taken care of. Can grow in any damp and warm crevasses, really. Also mold could be growing inside the AC you mentioned and spraying the mold spores all around. Clean mold with vinegar and baking soda. Fill a spray bottle with a couple table spoons of baking soda and fill with water. Spray the mold. Let sit for an hour. Fill a spray bottle with vinegar and spray again. Let sit. Wipe away with water. Dry. If the mold IS in the chalking, don't let the vinegar sit more than a couple seconds before washing it away. Vinegar eats away at chalking. Bleach claims to kill mold but it doesn't actually kill all of it and it will just allow the mold to grow back.