Can mold/mildew have this effect?

Yes, some people are not very good with mold/mildew at all. My neighbor is quite allergic to it and fights it by spraying hydrogen peroxide everywhere and wiping it down.

It could be mold/mildew, or perhaps something else you’re reacting to. Mold/mildew don’t typically smell lemony to me.

Are there any jars or jugs of paints, solvents, cleaners, etc. that might be leaking fumes?
Are there any gas appliances that could be giving off carbon monoxide?

My uncle has similar reaction to mold/mildew. See a doctor.

@hearkat Nothing else in there.

Next day, actually even earlier than that and I was feeling fine, so I just won’t hang out in there lol.

@XOIIO – Wow. I’ve never heard of nor seen a garage that didn’t have anything in it, like paint cans, turpentine, weed-whackers, lawnmowers, etc. Was it renovated into a den or something?

@hearkat Nope, just used for storage of my mothers random stuff, and half of it is my server and shitloads of random electronics bits and bobs I never tough lol. Lawnmower is kept in a small wooden shed. The only paint are one or two spray paint cans but those are intact.

There is also a heckuva lot of dust in there, isn’t there? Breathing that will cause those same symptoms.

@Dan_Lyons Barely any dust actually, a window is open all the time, usually the air is nice and fresh, but the whole roof seems to have gotten mildewey this year.