Hi. My Son Has A Weird Bruise On His Arm

Hi. My son has a weird bruise on his arm that doesnt hurt. Its got a white bump in middle.and a.reddish brown longish ring around it. Doesnt itch. Last week i had similiar bruises on my arm and leg but mine were a bit more red thanbrown and more round than long. Appeared out of no where. What is it? I have pics