Anatomy. Regardless of the reason (cold, allergies, etc), the reason your nose gets stuffy is that there is inflammation in the lining of the your nose, which then gets thicker than when there is no inflammation. The main component is from a structure called the inferior turbinates - these get enlarged, engorged with blood and thickened when there is inflammation ---> stuffy :).
Congestion. The nose is extremely vascular and there are structures in your nose called "turbinates" that are mostly blood vessels. Anything that angers or upsets your nose causes these blood vessels to fill up becoming "congested." whether from an allergy, a cold or just being in a dirty environment your nose responds by increasing blood flow. That's why we take decongestants to shrink the blood vessels.
Allergens/ vasodilat. Because allergens ( dust, mites, pollens, & others) and because vasodilatation from medicines, local warm temperature.
Why does my nose get stuffy after being in air conditioned room? Mainly in rooms with window ac units, not places with central air
? Mold allergy. Molds tend to linger more in window units/split units, which need cleaning more often, that can be a cause. Another cause can be vasomotor rhinitis, difference in temperature/atmospheric pressure would cause congestion in nasal vessels leading to stuffy nose. Keep using the saline nasal washes, try to see an ENT for evaluation if you really suffer. Read more...