Do you have any symptoms of infection, such as fever, chills, etc., before the onset of ear pain? Have you been tired recently? Usually, the severe pain behind the left ear is probably caused by herpes zoster virus stimulating the facial nerve. Mostly it happens when you have a low immunity. You need to go to a dermatologist and an otorhinolaryngologist to check whether there are small herpes behind the ear or in the external auditory canal. You can take vitamin B12 to nourish your nerves, and use gabapentin to relieve pain symptoms. Sometimes, you may also need to take a small dose of hormone.