Could it be heat rash? If its heat rash, wear something cooler n can put prickly heat powder. But it also could be Hormones. Changes of hormones may cause breakout or skin itch as well. You will have to see a doctor to get the right medication, don't buy it off shelves. :)
For me, is hive related but I not sure whether is hormones/allergies. So went to GP to see doctor. Just tell the GP you are preggy so that he could prescribe the suitable type.