What Causes Pain In Armpit Radiating To Shoulder Blade And Into The Breast?

I am a 30yr old female & have been having pain in my left armpit that radiates under my shoulder blade and into my breast for several weeks now. It started out feeling like my lymph nodes were swollen. I also have a pea size knot in my left armpit that does not hurt. maternal great great grandmother died from breast cancer paternal great grandmother had breast cancer 3 paternal great aunts died from breast cancer paternal melenoma paternal grandfather: melanoma, colon, & prostate cancer paternal aunt, rectal cancer