Breast pain 4 months after mastectomy

Flo80

Hi Friends I have done my radical mastectomy of left breast in November and radiation in February. I was done with chemotherapy in October. I still have some cycles of herceptin and perjeta left and also taking zoladex and exemestane. Now my healthy breast is Feeling tender when I press or lift it. No other changes in the skin or nipple. This is driving me insane