What Causes Red, Itchy Blotches On Arms And Legs?

i have these red itchy blotches, that have appeared on the back part of my arms, on the upper part of my legs only. Whst is this? I was taking and antibotic for sinus infection for about 5 days but I stipped because it did not seem to be helping and now this. I notice more blothes after i return from the beach?