capped tooth pain

[loudog800]

3 weeks ago I had some pain in a top molar that has a cap on it and was root canaled 5 years ago with no problems since. I went to a dentist fearing an abscess but he said the root canal looked great and he could see no problems with the x-ray. The pain lasted about 3 days and went away. However, it returned again the other day. It hurts when light pressure is applied although full pressure did not hurt.

What is causing this...my sinuses...the gum tissue around the crown????