Constant case of the sniffles or congestion? Try checking under your bed. I've admitted that I am not a great housekeeper, but I do vacuum once a week (twice during dog shedding season). And I move things around when I vacuum - picking up the rugs, vacuuming behind the tv's and under the chairs and even swiping off the air vents.
You should have seen my face, though, when I accidentally sucked up a sock from under my bed and stooped down to retrieve it. YEEIKES at the dust bunnies hiding under there!! I thought I had so much stored under my bed that a bunny couldn't fit! But there they were. No wonder my husband snores - he's sleeping on top of pet hair and dust and probably pollen from having the windows open.
So if you can't seem to find what's making you ill - try looking under the bed - you might find something worse than monsters hiding out!