What Causes Recurring Of Breast Cancer Post Chemotherapy And Radiation Treatments?

I had breast cancer, 2A in 2001, had eight chemo sessions, 33radiation treatments. Been fine ever since. Lately the CEA has bounced around from 0.6, 0.7 and 0.9. Six months ago was 0.6. Six months ago my CA 27.29 marker was in just in normal range (0.0-38.6), itwas31.6 CA 15-3 was high of 29.6 in range of o.o-25.0.
Now as of 1/5/15 my CEA is 0.9 again and the CA 27-29 is High at 41.4 (0.0-38.6) and the CA 15-3 is High at 35.8 (0.0-25.0) Doc wants to do PET Scan. What can be going on, I am very worried ??