What Causes Pain In Breast And Armpit Post Lumpectomy Surgery?

I had a lumpectomy 7 months ago, stage 0, 4 lymph out, mamosite radiation for 10 treatments Everything went well. This week I started having pain in my armpit and now thru my breast. Can t get in to see my doctor. Nurse said to continue doing my exercises. What do you think is causing this pain now?