Elevated CEA w/ history of CRC w/ neg CT

Hi -
I am a health professional who has a very wonderful mother who had a bout of colon cancer diagnosed July 2009.  I am really trying to understand the role of CEA elevation in surveillance for CRC recurrence.  She underwent neoadjuvant chemotherapy w/ Xeloda, w/ radiation to the rectum.  She had an 8 cm tumor at time of diagnosis.  PET/CT was otherwise negative at that time and before her surgery in February 2010.  She had "microinvasive" disease only at time of surgery, negative lymph nodes, and negative pelvic wash.  She underwent adjuvant chemotherapy (although her heme-onc said she didn't really need it).  She was a Duke's B, they gave her a 95% curative rate, although she had to stop the adjuvant chemotherapy d/t hand-foot syndrome.  Her CEA was never elevated until February 2011.  She now has a 5.3, which was repeated because it thought to be a mistake.  Her repeat 4 weeks later was 5.0.  She had a negative CT in October of 2010 (although she did have gallstones).  She just had liver US/gallbladder scan which just showed gallstones.  She also just had a negative mammogram.  
Her heme-onc is now reluctantly sending for a PET, he doesn't think it's going to show anything.
How worried should I be?