Lots of things can affect CEA. It's not a clear indicator of anything, just a clue to start looking for more data. I have the opposite problem. Mine never did elevate even when I had a big honkin' tumor in there. Now a low CEA tells us nothing at all. We still check them in case it SHOULD by chance elevate, but low ones are not as reassuring as they could be.
They're a rough assessment tool and nothing more. And remember too, that a recurrance, should it happen, is a setback. It isn't necessarily a death knell.