Suggest Remedies For Pain In The Back Of Legs

My 10 year old daughter is taking antibiotics for strep throat. She is on the 3rd day of antibiotics. Last night she complained to me about very bad pain in the back of her legs, in the top of both calf muscles when she tried to straiten her legs. she said she had had this pain for a few days and it was getting worse and worse. I had her drink water and go to sleep. This morning it was just as bad or worse. She can't walk without being on her toes and keeping her legs bent.