I've been getting something like this for years, but it does seem worse since getting pg. I have no idea what causes mine, but it starts with an itch, which I can't help but scratch. That then turns into red (white if you push on it) lumpy bumps which itch like crazy.
I was taking an antihistamine once a day to keep it at bay until bfp. Now I am still taking the tablets - Benadryl or similar (with doctor's say so) but as infrequently as possible. It's working out to perhaps one every other couple of days. I hate taking them, but I cannot survive with that rash. It really is completely overwhelming.
I get it on my hands, back, bum, legs, stomach and feet. Ugh.