This is more likely to be a UV allergy.
If you look closely and see that there is no rash on the inside of the sleeves and arms, then this is further confirmation that it is due to sun exposure. There can be two scenarios, one where one is allergic to UV rays and the other where certain substances enter the body and are then exposed to sunlight to produce an allergic reaction, these substances include some foods, medications etc. The rash may fade on its own after sun protection.
If the itching is intense, you can visit a dermatologist and be treated under medical supervision.