The blood vessels on one side of the nose swell up with blood, on purpose, to close off that side of the nose to air
. This gives the cilia in the closed nostril a chance to clean out all the pollution and mucus. When it’s clean, it opens up and the other side is closed to repeat the process. The things that swell up to close your nostrils are called turbinates.
Keep this in mind when you are blowing your nose. One side will always seem more congested than the other, even if it’s clean.