Why would my root canal filled tooth suddenly be sore??

[Miladymilord]

I had a root canal about 6 months ago. Its suddenly very sensitive and sore

I had a horrendous abcsess which, I am not kidding, still gives me nightmares.

Surely the actual tooth is dead now??

Could you have food debris caught either side of that tooth ? Floss or use one of those intident tooth things to clean between your tooth/teeth.

I'm not a doctor or dentist but i think sometimes you have a site of previous infection that your immune system keeps under control: filled tooth, tonsils, healed over pierced ears, cold sore (are examples for me) - when you are run down/ tired/ stressed/ or have another infection/cold to deal with - your immune system can "slip" a bit at that site - letting it "flare" up.