Lymph node involvement under the sternum

I was diagnosed with IDC Jan. 2010. I had 4 rounds of AC and 4 rounds of taxol. Double mastectomy followed by 27 rounds of radiation (finished radiation Aug 16). Lymph node involvement under the arm (1/13 had cancer) and 2 lymph nodes under the sternum were cancerous. Just wondering if anyone else has had lymph node involvement under the sternum. PET scan came back clear (for all the nodes) before my surgery, but after the surgery the pathology report showed there was one node under the arm that still had cancer. I keep wondering if the lymph nodes under the sternum still had cancer. 6 weeks went by from the time of my last chemo until I started radiation and I keep thinking maybe they became active. So I'm wondering if anyone has gone through this. I was on Arimidex for a week and started getting back pain. Dr. told me to stop taking the med. It has been 2 weeks since I stopped taking the med. but the pain in my mid/lower back hasn't gotten any better and some days it seems to be getting worse. Hard to bend down, try sitting down, or getting back up. I wonder if the cancer has come back. Talked to my oncologist and was told I need to contact my regular physician and start there because she didn't think it was cancer. Needless to say I am worried. I would like a PET scan or MRI done to ease my mind but I don't know if that will happen. If anyone has any info I would greatly appreciate it. Thanks for your time.