Thanks for the additional information. If you are certain that you are feeling cold on this exact tooth, then there has to still be some innervation on the tooth. Some tooth pain can be non specific and feel like it is coming from different teeth than the perceived one. To be certain,take a small ice cube and touch it to just the root canaled tooth and see if it reacts. If it does, there is yet another accessory canal. If it doesn't react, then we have a different source for the pain.
Hopefully the endodontist tested your other teeth to determine if the pain is coming from an additional tooth. If this was done and no other tooth responded to the tests as being inflamed or dead, then this is a bit of a mystery.
The lower teeth each have their own individual nerves that connect to one large nerve that runs underneath the teeth. I can think of two possible scenarios that could cause the pain you describe if the other things I mentioned are not the cause.
1. During the root canal procedure, the a cleaning file was inadvertently pushed out the end of the tooth and into the main nerve that connects all your teeth. This main nerve is inflamed and not the nerve of the root canaled tooth, which no longer exists. If this is the case, the pain will gradually go away.
2. The pressure from filling the canals has cracked the root and it is split into two pieces. This will cause pain and will eventually will become infected and swollen.
I hope this gives you more insight into the pain your are still having. Please reply to my answer if you have any other feedback or questions.
Sincerely, ***** ***** DDS