After taking 1 g of azithromycin for chlamydia, I feel better but on the 8th day my vagina got itchy. What could this be?

Maybe yeast. Possible a yeast infection related to recent antibioics. Trichomonas is another infection to consider, especiallly if you were not tested for thhis when your chlamydia was diagnosed. I advise you get a repeat pelvic exam.