What Causes Painful And Sore Breast?

Hi: I m a 70 yr old retired female. I have been pretty healthy; try to eat healthy and exercise at least 3 times a wk. This evening my breast started to hurt and became sore out of the blue. On occasion in the past I have had this sensation and it would normally go away. I do have my yearly Mammagrams and are all with in normal limits. So why does this happen?