Please give a better description of pain. Is it constant or intermittent? How long has the gum been paining? Does the pain worsen on hot or cold food or at night? The pain could be due to one of any reasons: A) A cavity in your left tooth B) Gum disease C) You might have cracked a tooth If you can give a better history, it might be possible to give a more precise answer.

it is gum disease called periodontitis..it must be treated as soon as possible.otherwise ignorance may lead to more damage to gums n bone..consult dentist