I have a cold, why is only one nostril affected?

You may have a sinus (bacterial) infection on one side. If you have a cold (viral) infection give it time, it will spread.

---

it could be the sinus infection, but also if only one nostril is affected anytime you are stuffed up, if could be that you have a deviated septum, the part of cartilage that separates your nostrils within the nasal cavity. it can be fixed with same day surgery(I had the same thing done and now i breath much easier and being stuffed up is not as bad.)