You probably have a PUPPP rash. You can ask your doctor for his suggestions on how to help the itching.
I had it with my oldest dd who is now 14. Back then my doctor said the only thing I could do was baking soda baths. I hated taking baths so I filled a small container with water and lots of baking soda then use a wash cloth to put it on the affected areas. I would get up 2 or 3 times a night to do this. I would wake up in the morning and would be powdery white.
The rash went away about a week after I had my dd. While labor, I had an epidural. I could still feel the itching but the epidural prevented me from feeling anyone scratching my legs for me.
I hope you find some relief. That was the most miserable part of my first pregnancy.