My gum hurts really bad on my left side and feels like a piece of the left side of my check is coming off, what's wrong?

No way to know: There's no way to know without personally examining you. You will need to see your dentist to know what's causing your pain and what can be done about it.

Gum pain: Could be a third molar erupting if it is at the back of the jaw, could be a gum abscess from a dying tooth. As we cannot see you the best thing is to see your dentist asap to get a diagnosis.