My CEA Level Has Been Climbing, First Slowly, Then Faster

My CEA level has been climbing, first slowly, then faster the last several months. It doubled from 5.3 to 10.5 from September 2016 to December 2016. I just went back to my doctor this week and it had risen again to 12.2 in less than a month. I had a CT scan last month that didn't show any recurrence. So she scheduled a PET for March 3rd. Should I be concerned about my CEA level? I had colon cancer stage 3C, surgery, 4 rounds of chemo was all I could tolerate, and 6 months of Xeloda. Was NED June 2016. My CEA has been steadily climbing, but nothing showed up in my CT.