I guess the first question is - has your partner been treated? Have you had any new partners since your treatment? If you have a partner - has he/she been faithful?
From what I have read, Zithromax is like 97% effective. Usually the only reason it doesn't work is if you didn't take the full course, or had unprotected sex with an infected partner during treatment.
Good luck, I'm sure the meds will work this time, if not your doc will give you something new to try.
PS - what were your symptoms? Im just curious to see if if I had the same.