What is this brown spot near the gumline of my tooth?

Hi, I have had a lot of dental issues in the past when I was younger I didn’t take care of my teeth, the past two years I’ve had a lot of work done on them and have been taking care of them quite well. Today I noticed under one of my back molars there is a brown hard spot (kinda tan colored) under my tooth right on the gum line. It hurts when I touch it with my dental pick but otherwise the tooth doesn’t really cause me pain. The tooth also has a amalgam filling near the gum line and the brown spot is directly under it. Could this be decay possibly a cavity? Or would this result in a root canal. I’m going to the dentist on the 10th but I would like to know what to prepare for. I’m hoping that it can just be filled and possibly crowned. If anyone could help I would appreciate it! Thank you!