Any idea what is causing my tooth to hurt?!

I went to the dentist the other day for a cleaning and the only bad part of my report was my gums were a little inflamed. But yesterday I got really bad pressure on one of my teeth that I can't relieve. There is a filling on top and it doesn't looked cracked, but I whenever I bite down is a dull deep pain. I just had xrays and they didn't mention anything. Ideas?