My daughter had this too. Her cheek swelled up and she was in dreadful pain. The doctor didn't know what it was so I made an appointment with a neurologist because I thought maybe it might be neuralgia of some sort.
As soon as this doctor walked in she said, oh, that's not neuralgia, but we'll take care of it. So we eventually ended up in the ER and they had an oral surgeon come in and she had an infection. It was pretty gross the stuff he sucked out of it. But, it took a lot of antibiotics to finally get rid of it. The dentist had even taken x-rays.
It was apparently hard to diagnose but you should go see a doctor or dentist and they need to take an xray. It will probably not go away and will get worse before it gets better, even if it does.