OMFG my tooth is KILLING me!?

Last year the dentist said I have good healthy teeth but that I have an issue with clenching my jaw in my sleep and even while awake sometimes- There's one specific tooth that started hurting awhile ago and got progressively worse, then the other night I was eating a carrot and this horrible sharp pain shot up my jaw. It was so unbearable that I had to go to the emergency room, where they gave me painkillers, then I went to a dentist who did an x-ray and couldn't find anyting! How is that possible? I'm in agonizing pain and it's not letting up at all I don't understand :-(