Rash...

So all of a sudden developed a rash.. It's on my stomach arms and legs and feet. It has been driving me up a while these past couple of days.. I'm 24 +3... I called my obgyn and they told me they don't do "skin" so I called and made an apt with the dermatologist .. I went yesterday and they basically said they can't do anything for me.. Just to put benadryl or drink benadryl... Well problem is I'm not going to drink benadryl for the rest of my pregnancy I don't care how safe they say it is I don't think it would be considering it's a man made medicine. Anyway. They also said they Don't know what it could be. I've heard so many women say it could be pupps rash or cholastatis. How would I know?? I feel like making an appointment with a regular physician .. Like how can an obgyn tell you they don't know the dermatologist say they Don't know aNd I'm out of luck due to pregnancy.. I'm so upset.