A nasal congestion in only one nostril is a physiological phenomenon, and the side that is affected alternates periodically.

Although we have two nostrils, many people have probably experienced feeling like one nostril is blocked even when they are not sick. In fact, a natural 'nasal cycle' is at work in which the nostril that is easier to pass through and the one that is more difficult to pass through naturally alternates. According to Adam Taylor, an anatomist at Lancaster University, this mechanism is an important physiological phenomenon that allows the nasal mucosa to rest, prevents dryness and irritation, and supports the function of warming and humidifying the inhaled air.

This is why you only breathe out of one nostril at a time
https://theconversation.com/this-is-why-you-only-breathe-out-of-one-nostril-at-a-time-276407

Feeling that one nostril is blocked while the other is clear is not necessarily a sign of an abnormality or illness. According to Taylor, the nose has a natural 'nasal cycle' in which one nostril is clearer than the other, and this is a normal physiological phenomenon that protects the nose and respiratory system. During the nasal cycle, one nostril becomes congested and airflow is restricted, while the other becomes decongested and more air can pass through.

The nasal cycle is unconsciously regulated by the hypothalamus in the brain, and while awake, it switches at a minimum of about every two hours. However, during sleep, breathing slows down and the amount of air entering and leaving the body decreases, so the frequency of the nasal cycle also decreases.

The nasal cycle is important because the nose is the first line of defense against pathogens, passing at least 12,000 liters of air through it each day. By alternating which side is dominant, it prevents one side from becoming more susceptible to dryness and irritation, allowing the mucous membrane to have time to rest and repair itself. Furthermore, the congestion process increases blood flow to the nasal blood vessels, which not only helps to keep the mucous membrane moist but also warms and humidifies the air passing through the nose.

Past studies have suggested that when the right nostril is dominant, the body is in a more aroused or stressed state, while studies have suggested that when the left nostril is dominant, the body may be in a more relaxed state.

However, this nasal cycle can be disrupted by various factors. For example, colds and influenza increase mucus production, and allergies such as pollen and dust mites cause severe inflammation of the nasal tissue. Medications that act on blood vessels, such as those for high blood pressure, can irritate the nasal mucosa, and using nasal sprays to relieve nasal congestion for more than five days can cause drug-induced rhinitis, negatively impacting the nasal cycle.

Nasal congestion can last up to two weeks for colds or the flu, up to four weeks for sinusitis, and several weeks for hay fever. Furthermore, structural problems such as nasal polyps or a deviated septum can disrupt the nasal cycle, making both nostrils feel blocked. Even simple actions like lying down or hunching over can alter nasal airflow due to the uneven distribution of blood and sinus contents.

Taylor prefaced her remarks by saying that 'there's no need to be overly concerned about having one nostril clearer than the other; it's a normal physiological process to protect your nose,' but added that caution is needed if the condition persists for a long time. Taylor said that if one nostril is blocked for more than two weeks, or if the appearance of nasal discharge or secretions is different from usual, you should see a doctor.