I've been having hives for about three days already, I went to the doctor and he said I had an allergic reaction but I've never had an allergic reaction and I didn't know I had even had one. He prescribed me with Zyrtec and I'm taking one everyday but they keep coming back after about 4 - 5 hours. When I go to sleep, I wake up in the next morning with hives all over my body again. What's going on?