Suggest Remedy For Itchy Rashes On Being Exposed To Sun

Starting around April of this year (2011), I have been getting a rash (tiny red bumps), on my arms (from wrist to elbow) and on my neck whenever I am exposed to the sun for a period of time. I have used sunscreen but still got "sun" when at the beach, waterpark, hiking, etc... It is very itchy but "hurts" when I rub/scratch it. What is it and why is it happening to me now (I am 26) even though I have lived in Florida, Guam, etc... for all my life? How do I stop breaking out?