What Causes Severe Itching And Redness All Over The Body?

I have severe itching and redness all over..but the major outbreak of hives is on my trunk and upper legs. I am taking bactrim for mrsa and already had to be pulled off of topamax this week for adverse reactiosns. My hands and around my eye also swollen and ache. I was told to quit taking the bactrim and no further info. Antihisttamines are not helping..idk what to do.