What Causes Pain In The Left Breast?

I am a 44 year old female. I am having pain in the upper right side of my left breast. I sometimes experience back pain under my left shoulder blade. It only hurts when I take a deep breath. I don't feel or see anything different in my breast. If I put pressure on that spot when I take a deep breath, it seems doesn't seem to be as bad. Most of the time it is just more uncomfortable than hurting.