Promethazine symptoms and conditions

I had a skin infection on my lower legs and was prescribed Cephalexin Monohydrate. I took it for 6 days, 4x per day, then stopped. I was supposed to continue on 2x per day for the next week.
I started itching at the top half of my buttocks and it quickly spread all over my back, chest, shoulders, arms and onto my scalp. Small lumps or hives and red blotchy skin nearly all over my old body. The itching was driving me crazy. I didn't know where to scratch or rub next.
My doctor couldn't see me for another week so I went to see another doctor who prescribed Phenergan (promethazine hydrochloride) tablets and Celestone M (betamethasone) cream.
Relief came within 24 hrs, I went through 4 days of hell and while still itchy on the 5th day, it was getting better. I also used ionil T ( a medicated shampoo ) for my scalp.
I have never been allergic to any drugs before this monster
This antibiotic should come with a warning