Hi, You have a typical history of pain or stress under left ear and chin more so while walking and sleeping, feeling as if left digastric muscle and tendon is stiff. The commonest cause of such symptom is called Eagle s syndrome wherein there is inflammation, spasm or bone deposits in stylohyoid ligament. The exact cause of such a problem is not known. This may be just an inflammation thus the strain and pain. The diagnosis depends upon suspicion and relevant clinical examination, MRI of the affected area. The treatment is an anti-inflammatory with painkiller properties like Ibuprofen three times a day as pre your weight. Additional treatment will be as per the prescription of your Doctor/ENT Specialist. Complete the investigations as late diagnosis can miss important points. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. T. Chandrakant, General Surgeon