My symptoms also started in September of 2011 after several tick bites between the spring and summer. With my CD57 being under 20 and some of the minor symptoms that I had for may years prior, several LLMD's thought that it was likely I had Lyme for years and was reinfected last year which is why the symptoms intensified suddenly.
I was skeptical too at first, but I think the CD57 should make you more comfortable with the diagnosis. Some doctors believe that only the Lyme bacteria (Bb) can lower the CD57 count. Generally it's useful if the number turns up low (for validating Lyme) but if it is in the normal or high range, it doesn't necessarily mean that you DON'T have Lyme because other factors could raise it like certain co-infections.