How does a stuffy nose occur?

Swelling & mucus. A nose gets stuffy because the inside lining (the mucosa) of the nose swells up, blocking the air flow. The lining also oozes mucus, which blocks airflow even more. Stuffy noses can happen with colds, with allergies, or sometimes with no apparent reason. Blowing one's nose pushes the mucus out, but the swelling would still remain.
Allergy / infection. Can be due from allergy or infection , but best to see your otolaryngologist for exact diagnosis and appropriate treatment.