I had it - lasted for a few days and then went away on its own. The doc told me it's normal for pregnant women to have reactions to detergents, food, etc, that they have never reacted to before. Use aqueous cream to keep your skin moisturised (dryness will make the itching worse) and calomine lotion for the itching. Also your stretch mark cream should help moisturise too. Hope it goes away quickly!
Had these rashes for both my first pregnancy (and now also to a lesser extent in my current second.)
First pregnancy I got the big flat urticaria types all up and down my legs, which I scratched to riibbons unfortunately, and the tiny pin prick types actually in the stretch marks all over my belly, so from the boobs down, my entire body was red and angry and as you say, tortuously itchy!
First time round, gp prescribed a (weak) hydrocortisone which was so-so. This time round I'm prepared....
...bio-oil in VAST quantities, about 3 times a day, plus I'm hacking at our aloe vera plants in between which just takes the bite out of the worst of the rash. All this has kept the rash to just around my belly and so far is not as bad as previously. I don't care about the ugliness of stretch marks (they didn't look half as bad after the birth anyway), its just the itchiness I need to avoid.