What Does A Hard Lump On Collarbone Indicate?

I discovered a hard nodule on my collarbone just off center of the middle of my neck. Over the holidays, I've noticed it has grown twice it's size. I saw a doctor mid December regarding something else, but showed both my heart doctor and primary doctor, and they suggested to "watch it" for a month and reevaluate. But I'm worried. I am 66 years old, female, sedentary due to a heart situation.