Hi there, It might be possible that the antibiotic has triggered off an episode of vaginal candidiasis, or yeast infection. Is there any discharge? Typically candidiasis causes a thick whitish discharge, and is intensely itchy. It can come with or without a rash, depending on whether the outer skin is involved. I would recommend a vaginal pessary of an antifungal like clotrimazole, or if you prefer oral medication, something like fluconazole would help. Usually a single dose of either one would suffice, but some preparations of pessaries will require a three day course. I do agree that cephalexin is in the same group as Rocephin, or ceftriaxone, but some people respond differently to medication that are in the same group. Because the itching is quite delayed and localised to the vagina, it is unlikely that it is an allergic reaction to the injection. Hope this helps, and please let me know if you have further questions. Regards, Dr. Teh