What Could Itching And Fast Spreading Rash During Pregnancy Indicate?

I'm 15 weeks pregnant, 28 years old. A week ago after spending an afternoon outside, with a tight athletic tank top on, my back began to itch. After a couple days I looked closely and noticed pink/light red clusters or blotches of a rash, randomly on my back. The itching continued and throughout the week the rash has spread to include parts of my shoulders, neck, chest, and stomach. It's been a week and the itching seems worse than before. There is no pus, I have stayed in air conditioning, and have used benadryl and calamine lotion. Nothing seems to help the itch, and each day "splotches" seem to spread, and it's non-stop itching. Unsure what the rash could be, or how to treat it since I'm pregnant.