Why does cold air smell

Why does the air usually smell different when it's cold?

I recently moved and added some furniture in my bedroom and since then it doesn't smell very fresh here. The smell is not necessarily unpleasant or disgusting, it "smells" stuffy, as if one would never ventilate here.

The window is open almost all the time, maybe three or four hours a day. However, you only get a bit of the fresh air if you stand directly in front of the window. The door is usually also open, so there is a draft.

I have a rug that is only a year old and smells pretty neutral.

There are also curtains that have only been hanging here for less than a week. They were washed and aired out.

My bed has a metal frame, the mattress is about 5 years old but is regularly aired outside. The same applies to the bed linen, which is of course also washed.

In the room there is otherwise a desk, a computer, a television, a wardrobe and a few shelves, all of which are regularly dusted / wiped with a damp cloth. The devices are on for maybe 2 hours in the evening, but not all day long.

Since I recently moved the furniture, as mentioned, I was also able to clean behind the furniture and I didn't notice anything because of mold or the like.

The house itself is about 30 years old and was completely renovated 10 years ago. The room is about 15 square meters and has only one window.

I don't know now what the problem is and what to do about it, is the humidity too bad?