pain after root canal and a crown

at my last dental check up the xray showed i had a small crack in my front tooth at the bottom of the tooth no  where near the gum line. the dentist suggested a root canal and a crown. that was in dec of 2010 . i did have both done and since the time the crown was put on ive had pain in the tooth which i never had before, it hurts  to bite down on anything and i have a consent dull pain going through the tooth and it hurt to touch the tooth. ive been back to the dentist 4 times and he keeps saying the tooth is going to calm down at so point  but its now been three months. any ideas on what going on. could the dentist have cracked my tooth while doing the root canal or could the crown be had wrong. thabks for any opioions