I Am 53 Year Old Post Menopausal Woman With Unilateral

I am 53 year old post menopausal woman with unilateral breast swelling ( or enlargement ) on right side. No pain or physical abnormalities associated. Had mammogram done 3 months ago and came back normal as did breast exam done by PA. Mother did have full mastectomy on left breast and made to 5 year mark but metastesised to brain and lung; so you understand my concern. What could be cause.