Looks like reinfection of the root canal. In most cases of post and core buildup there are chances of root fractures.t hey are difficult to diagnose in x-rays

Sometimes there are anatomical and technical issues in a root canal that can cause reinfection of an already treated tooth. Also sometimes if gum health is not maintained the infection can be from the gum to the tooth called in dental parlance as perio-endo lesion. Your dentist will get an X-ray done for that tooth and evaluate the correct reason for it. If it's a missed canal(anatomical difficulty) it can be re done. If it's a gum issue, that can be checked as well Sometimes a post and core treated tooth might fracture due to uneven forces. Let your dentist evaluate the right cause.