I Had A Tooth Pulled When I Was 21 Years

I had a tooth pulled when I was 21 years old. I am now 35. Ever since I had it pulled I get a pimple that shows up where the tooth was. It only hurts if I eat something hard that goes right down in that empty socket and really hits it. It will stay a few days or weeks then be gone for weeks or months. What could this be? Should I be worried? It's been going on for 15 years now. Never gets worse. Always same spot never hurts.