I would call the dentist you just saw and insist they see you right away.It is hard to believe it wasn't cracked when they did the xray, but it is possible it didn't show up if it was very minor. Chewing on it may have been all it needed to crack wide open. You need this seen to right away before the nerve is exposed or infection.
Of coarse the decision is up to you, but if it were me, on second thought, I would be there when they open in the morning. :|