Well if its that bad you will need to see a doctor, as if its an infection you will need treatment. Just tell your mum how much it hurts and im sure she will take you to the doctors, we wont be able to tell you what it could and couldn't be really.
Do you mean directly below your ear as below the root where it joins the skin of your neck, or below the bottom of the auricle? Either way just behind the jaw bone is a susceptible place with a mountain of nerves and arteries and you really need to get it checked, especially if it's hurting, and even more especially since it's hard.
I had a swollen lymph node behind/under my ear. It was rock hard, smooth and painful and every time I get a cold it swells up. So yeah that's probably what it is. I was freaking out and my auntie (who is a doctor) took one look/feel of it and told me what it was. Better get it checked though just to be sure.