Re: suddenly sensitive to heat 5 years after root canal???
I suggest you go to the dentist soon and have an x-ray done of the root tip so they can check for any possible abscesses. I don't want to scare you or anything, but I had the exact same symptom: sensitivity which turned to pain when eating or drinking anything hot. It was on a molar that had been root canaled a year before.
It turned out that a) a canal had been missed and b) the root had abscessed. Mine ended up flaring up so fast, so I suggest not putting it off. It could turn out to be none of the above, but better safe than sorry.