I'm not a doctor obviously, so if it doesn't go away in a couple of weeks even if you don't overheat your premises and open the window from time to time and take regular walks outside, then it might be useful, as the others have suggested, to check with a real doctor.
All I'm saying is: don't panic too much about it, had a similar problem five years ago and after two months it just disappeared and never returned - it was also in November/December.