What Does An Increase In CEA Level Post Chemotherapy Suggest?

I was diagnosed with colon cancer in Sept. 2014. All of my pathology results showed that no cancer spread. 39 nodes were removed- no cancer cells, no cancer outside of the tumor on the ends of the cut part of my colon. I lost my spleen in surgery and no cancer cells were identified when the spleen was examined. So... I was diagnosed with state IIa. I have just completed 6 months of chemo. The only reason I was on chemo was because I had a total blockage. I am scheduled for a colon resection in three weeks. However my CEA level went from 3.7 (right before chemo) to 6.4 (my last blood test). My oncologist assures me that this level is low especially since I am a smoker. She is concerned that it went up and so is scheduling another blood test and a CAT scan before my surgery. I am very worried that something might have been missed initially. What are the chances that the cancer is back?