Around six weeks I started getting rashes and I asked my dr he said it is bc the hormones make your skin super sensitive. I am now 31 weeks pregnant and still getting random rashes. There is no rhym or reason behind them just random rash on my hand or tonight one on my left butt cheek. I had to invest in a Sam's club size bottle of benadryl and a four pack of cortisone cream bc I use it nearly every day! Also if you get super itchy put and ice pack where it itches or soak on a warm bath of Epsom salts (you can buy this at any pharmacy, Target, Walmart, Walgreens, CVS). Sorry you are so itchy, you aren't alone though!! I can't shave my legs bc the rash gets so bad that I scratch in my sleep till I bleed. Ah the joys of pregnancy ;)
It could be because of your hormones going way up and the changes the baby is making to your body. It's better to ask your dr since I'm not of course just to make sure but I'm sure it's nothing serious =)
Congratulations on the baby by the way <3