Hello, I'm 15, And On Sunday While In Church, My

Hello, I'm 15, and on Sunday while in church, my boyfriend told me I had a red mark on my arm....I hadn't noticed it before that. It is on my upper arm and is about 3-4 inches long, and it hurts, and there are kind of circular areas on the edges. I was wondering if any of you guys knew what it may possibly be.