Redness Swelling Upper Left Arm, It Is Warm To The

Redness swelling upper left arm, it is warm to the touch, swollen, very painful and know it is starting to itch, and now I am getting the same thing in my abdominal and left breast. I went to the dr on tues he put me on 500mg cephalexin 3x a day. I have been tacking that but feel worse tonight with the pain an itching in left arm, aches all over and nausea. I also notice my arm is getting deep purple under skin. I have had mersa in my lungs about 4 yrs ago, was hospitalized. I am getting worried thats whats happening not sure what to do if I am just over reacting