I have had pain in my back right bottom tooth but it had a root canal. I grind there so is it sore root?

Could be fractured. You don't say if the back tooth has had a crown on it, but after root canals, molars can break if you grind on them. Dentists put crowns on these teeth to protect them from fracture after root canals. Get this checked by your dentist - a fractured root hurts in the jaw bone, not in the middle of the tooth.
It could. Teeth like any other part of our body can bruise and get sore after an injury. I would blame on grinding if the pain is mostly in the morning or after chewing hard food. Now, if the pain is spontaneous and/or lingers it could be a reinfection or a fracture causing infection.