Please Help - X ray of root canal tooth shows infection - Xray attached

I've posted a few times but sadly I haven't been of much help to anyone but this forum has been very helpful. Thank you everybody, 

Well I've posted the timeline of my onset and fight with bad breath. The link - http://curezone.com/forums/fm.asp?i=2116749

Another development was that the gums around the tooth( 2nd last molar on the left side bottom), I had root canal on bled recently (1st occurrence). The root canal happened on 11th of July this year. The tooth hasn't been capped but it already shows signs of decay. The permanent filling has a big crack in the middle and signs of decay on top of it and the sides.

I've got that root canal checked twice by 2 different doctors besides the dentist who did the root canal and both of them said that the root canal is fine. One even took an X ray and said nothing wrong.

I took out the last X ray done on 11th October and noticed that there is a dark area around the right side of the X ray. 

My previous panoramic x ray done on 10th July shows dark area near both the roots but the one on 11th shows only on side of the tooth. I would like a opinion of curezone members because I seem to get better opinions here than by doctors.

I'm attaching the X ray.