What Causes Scar Tissue In Breast Post Lumpectomy?

I AM CONCERNED ABOUT SCAR TISSUE DISCOMFORT MY DOCTOR SAID IS MAING MY BREAST SO TENDER IN MY RIGHT BREAST WHICH I HAD A LUMPECTOMY ON 6 OR 7 YEARS AGO. AT TIMES IS FEELS LIKE A LUMP IS THERE BUT EACH TIME THE DOCTOR CHECKS IT, THERE IS NO LUMP FOUND, THANK GOD AND I ALSO HAD THE MAMMOGRAM AND ULTRASOUND DONE WITH NOTHING FOUND. WHAT COULD BE THE CAUSE OF THE SCAR TISSUE BOTHERING ME NOW, IF IT IS IN FACT SCAR TISSUE?