Question about upper right tooth pain~~

About 10 years ago in Changwon, I visited the dentist because of tooth pain for no apparent reason. I don't remember exactly, but they left the root of the tooth and placed a new tooth on top of the root. Recently, I sometimes have phlegm build up and when I cough it up, I experience pain in the treated area... Could you tell me why this might be happening?

I'm leaving this message because dental treatment costs a lot of money, so I wanted to ask if I could get some information~~