What Causes Recurring Red Inflamed Rash?

I get a rash on a regular basis for the last 3 months its red and inflamed itchy at the beginning and hot it lasts for 24/36 hours at a time is very painful and as it goes away i end up with a yellow coloured bruise. Went to the doctor several times and have got several different answers. What could this be as i am getting worried. Could you please help me?