Diagnosed With Breast Cancer That Has Invaded My Bones 3

Diagnosed with breast cancer that has invaded my bones 3 years ago. Been through chemotherapy and radiation. Breast cancer is no longer but bone cancer is still spreading. Just changed to BRAVCAP or Capecitabine from Afinitor and Teva-exemestane. Have an Xgeva injection once every 6 weeks. I have blood tests every month and my Tumour Markers have been on the rise since the chemo. The last test has come back with the CA125 at 45 and the CA 15-3 at 920. Why are these markers so high? All tests are performed by the same laboratory. My oncologist says he is baffled by the steady rise. Any thoughts as to what might be causing this would be appreciated.