Broken tooth didn't hurt, now it started hurting - why?

[Lioness]

Good evening everyone.

I broke my another tooth last year in September. I guess it's #5, upper. I suppose it was done by root-canal in the past. It didn't hurt after being broken so I let it be. Now it's started hurting - why?

 

[Gordon]

Hi Kitty.

Bacteria have got into the root canal system via the fracture. They've spilled out of the apex of the tooth and set off an immune response at the apex. This is causing a build up of pressure in the tiny space between the top of the root and the bone, it should hurt more when you bite on it or if you drink something hot. Cold things will make it better for a while (I think that'll be Charles' Law in action? Courtesy of my Physics teacher in 1973)
Sometimes repairing the tooth and sealing up the root canal system will sort it, otherwise the root canal treatment needs redone or the tooth removed. Sorry if that's not what you want to hear.