Itchy rash on my arms that spread burns when i scratch it?

Contact allergy: Can't say for sure - two clues. Itching (histamine-mediated which often implies allergen) and spreads with scratching (often a substance that is carried on the fingers). Poison oak, ivy and sumac are the archetypical contact allergens; there are hundreds of others. Treat with gentle soap and water wash and cleaning everything which contacts it. Antihistamines and over the counter steroids optional.

Hives: A rash that spreads when you scratch it is almost always hives (urticaria). (poison ivy and other contact rashes do spread, but not when you scratch them.) lots of things cause hives, from foods to drugs to stress to most anything else. Try a non-sedating antihistamine like allegra (fexofenadine). If that has no effect, you'll need to have a dermatologist look at the rash.

Many possibilities: There are many things that could cause the skin to itch. 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.