I have a pain located on the left side of the back of my neck. it feels like an hematoma but i haven't been injured. what could it be ?

Get it checked: Many causes, could be a lymph node or collection under the skin like Lipoma (sac of oil)

See doctor : It could be a lymph node or boil or sebaceous cyst or lipoma. Please see your doctor from r examination to confirm the diagnosis.

Strain?: best to get evaluated. He/she will be able to tell you if you have enlarged lymph nodes? A tumor? An infection ? Tissue swelling ? Muscle strain. Start with a family or primary MD. if beyond their scope they ma refer you to a neck specialist