Biopsy. If you have a persistent rash, that has not responded to treatment and have no definite diagnosis - you likely need a representative biopsy taken for the diagnosis. If your primary physician can't do this, you should be able to make an appointment with a dermatologist.
Have it biopsied. There are many types and causes of skin rashes. Several types of inflammatory conditions of the skin, allergic reactions, and sometimes bacterial, viral or fungal infections can cause rashes on the skin. Insect bites can also be the cause of a rash on the skin. Have it evaluated by a professional and get the appropriate treatment.