As others suggested, you most definitely need to check with your OB/Gyn. Rashes can be indicative of bigger problems, so please don't wait on this. Even if it is "just a rash" your doctor will likely be able to prescribe something to relieve your agony.
I had a horrible rash during my 2nd pregnancy, too. It covered my arms, legs, and back. It wasn't PUPPS, but it was absolutely miserable. I itched so badly that I would cry out of frustration, and I couldn't sleep. When I did fall asleep I would unconsciously scratch until I bled. I had bumps and scabs and scars all over me. My skin didn't even feel like skin. I had no idea a simple little rash could be so debilitating.
Like one of the other moms said, I tried oatmeal baths and every cream on the market. Nothing worked. My OB/Gyn finally sent me to a dermatologist who prescribed Prednisone (an oral steroid). I was extremely hesitant to take it, but my OB/Gyn said my stress and lack of sleep were doing more harm than the drug would. Anyway, the Prednisone brought almost immediate relief, and my daughter (now 4) had no negative side effects from it.
My heart goes out to you, and I hope you find some relief.