Swelling and pain on one cheek can be caused by acute injury, mumps, interstitial infections and other factors. 1. Acute injury: when one cheek suffers an acute injury, such as an external blow, it will cause pain and swelling on one cheek, accompanied by redness and subcutaneous bruising. 2. Mumps: usually associated with mumps virus infection, which is more likely to occur in the pediatric population, can lead to swelling and pain in one cheek, which may be accompanied by fever and malaise. 3. Interstitial infection: usually associated with odontogenic infection, when the infection worsens, when there may be infection of the buccal interstitial space and other parts of the cheek, it can lead to swelling and pain in one cheek, which may be accompanied by swelling and pain in the gums and other symptoms. Swelling and pain on one side of the cheek is an indication of an abnormal condition and it is recommended that you seek medical attention for appropriate examination and treatment.