It is an L2 which is the "safer" category. My Hale's says: "The predicted dose of azithromycin (zithromax) received by the infant would be approximately 0.4 mg/kg/day. This would suggest that the level of azithromycin ingested by a breastfeeding infant is not clinically significant. New pediatric formulations of azithromycin have been recently introduced. ..."
I would keep breastfeeding while taking it.