What Causes Tender Breast Postmenopausal?

I am menopausal (69 yrs.) I have very tender left breast, sometimes pain down my left side and pain at back of left arm. I also notice a spot under may arm that has not hair growth. I have had a 2 blodd tests that shows prolactin level of 41.3 and 42.5. CT scan show pituitary is normal, but MRI has been ordered