Does too much time indoors make you sick?

I've recently had ACL surgery and I've been layed up inside for almost a week. I noticed I feel nauseous and have headaches frequently. I don't have the flu. I also noticed that when I open the door and spend a few minutes breathing outside air- or when a friend brings me somewhere, I feel better. Why? Is this related to being inside too much? If so, what is physiologically happening to cause this...?