Small Red Dots On Legs, Turned Larger, Black Scabs. Prescribed Keflax, Hives On Arms, Thighs. Causes?

Two weeks ago on both legs between the knee and ankle , hundreds of what looked like mosquito bites appeared , but was not iching, these then turned larger and then black scabs appeared. My Dr prescribed antibiotics Keflax , two days latter I broke out in hives on my arms and thighs. My Dr then prescribed Predinsone 20mg/day, some of the rashes have gone away but new ones have appeared. I have taken myself off oft the antibiotics. Now today a new round of the tiny red mosquito type spots have appeared on my upper legs. Any idea what this may be?