Opening in the side of my tooth where I got a Root Canal!

I can't drink cold drinks because I had a root canal on one of my back teeth and i looked on the inside and there was a dark line along the gum where the cap was put on!. Scraped, nothing came off and I think it might be a separation in between the cap and my gm!. Is this common!? I thought, though, that when you have a Root Canal they take the nerve out!. So why does it hurt anyway, if this is what it is!? Www@Answer-Health@Com

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