I work in an ER at a children's hospital and have had children with very similar symptoms during each of the last 2 shifts I have worked. The kids have several days of fever, one had even been treated for strep as in your case. They awoke with calf pain so severe they refused to walk, but it gradually improved during the day. Their lab work showed a low white count, low h/h, and low platelets with elevated muscle enzyme levels. With IV fluids both felt much better and we were left with the diagnosis of some sort of virus. Both have recovered well by report. Hope all goes well for your loved one as well.