What Causes Knot On The Center Of Breast Bone?

I an 72 yr old female and I have a hard knot on center of my breast bone, its about size of a quarter. I had a chest xray and echocardiogram recently by my heart doctor and they didn't say anything about it. I also notice a little soreness there and thinking it may be from the tech pressing on my chest during the echo. I am overweight mostly in my stomach where I carry most of my weight