Scratching will NOT spread the chicken pox. However scratching the pox will leave scars. The best thing you can do is put caladryl lotion on them and give her lots of fluids and basic meals like soup, scambles eggs, pudding, remember the pox can form in the mouth and throat.
It could also be a form of the measels. There are 5 types of measels and we only vacinate for 2 of them. Measels itch also.
It could also be hives. If it's hives give her benedryl liquid and let her sleep it off. They can last for days and up to 2 weeks. I have been through hives as a kid and with one of my kids and 2 grandchildren. It's one of those things that frustrates the daylights out of you because the only thing that works is benedryl and maybe an ice pack for the swelling. Hives can form in the mouth and throat and then becomes anaphlactic shock.