I wasn't familiar with cephalexin, so I looked it up, and it's an antibiotic. It will help her wound heal, but it won't help with the itching. I'd call your vet and ask if it can be taken with benadryl, I'm sure it'd be fine. Also ask about the dose of benadryl she should take. I would say two 25mg. pills twice a day as needed, but ask your vet first. I wouldn't go as far as getting a cortisone shot unless the benadryl doesn't work for her.

Another thing I've had VERY good luck with is Allercaine spray. It works great, I even use it on myself when I get a bug bite.