itchy

I know this is a rather grim subject but I need advice.  
I'm 25 weeks pregnant and ever since week 3 of pregnancy I have a really itchy vagina. I have been to the doctors and they tested for EVERYTHING and everything is negative. It's caused by increased vaginal discharge. 
I have tried sudocrem and vagisil to help soothe the itching but neither work. It's getting to the point where it's waking me up in the middle of night.
Any advice?