What Causes Red Rashes All Over The Body?

hi, I am having rashes and reddish skin all over my body since the last 2 weeks. I consulted a general practitioner and got medicine for allergy (allegra) but now the course is over and the rashes have come up again and they are more severe. It itches and burns a lot. I dont know whether its a skin problem or something else because I have slight fever as well. Whom should I consult in such a case??