Possible causes of left side neck swelling include pillow fall, cervical spondylosis, thyroid disease, heart disease and so on. 1. Falling sleep: If sleeping in an improper posture, catching a cold, etc., which causes static injury to the muscles of the neck and misalignment of the small joints, neck pain, soreness and swelling can occur. 2. Cervical spondylosis: neck swelling accompanied by neck pain, muscle stiffness, upper extremity pain, numbness, dizziness and other symptoms may be cervical spondylosis. 3. Thyroid disease: the left side of the neck swelling at the same time you can feel the thyroid gland is enlarged, pain, may be caused by thyroid disease, such as hyperthyroidism and so on. 4. Heart disease: the left side of the neck swelling accompanied by chest tightness, panic, shortness of breath and other symptoms, may be caused by heart system diseases, such as coronary heart disease. Caused by the left side of the neck swelling sensation of more reasons, may also be related to soft tissue injuries, etc., if the symptoms continue to be unrelieved, it is recommended to consult a doctor in a timely manner, to clarify the cause of the disease, targeted treatment.