What Causes Pain In Breast And Pectoral Area?

I am a 59yr old woman.Generally healthy and up to date with mammograms and not on any medication. I have noticed pains through my left breast and pectril area of arm intermittingly now for some months. I assumed it was muscle pain as i do carry a heavy shoulder bag on that side, also i play and lift my 2yr old grandaughter into the car often. what are your thoughts please?