hives while pregnant

Has anyone randomly got hives while pregnant? I'm 21 weeks and I've had them for about 32 hours now. They come and go but they are SO itchy. The OB on call told me to go to express care if they were still there after Benadryl yesterday and I did. That doctor said it just looks like an allergic reaction but they don't seem to be going away. I'll be going to my OB tomorrow hopefully if they can get me in..