Possibly an infection by anaerobic bacteria that remain in your sphenoid or frontal sinuses, as a result of the cold, messing with your olfactories, which are in the same area of your head. Here's the only cure that works, except if you just wait and it goes away on its own.
The flip-turn flush is mildly risky, because it requires that you bend over, preferably in a shower, but you can do it outside on soft ground. Continue to do it for about 3 days after the phantom smell goes away, or it will grow back, like it did to me 2 times until i figured it out. And give it a month before you resume smoking (gotta be addicted to something), because the smoke temporarily paralyzes the cilia.