Suggest Remedy For Bruises In Arm

Last week, I noticed a small dime sized circular bruise on my upper right arm with a lightened middle. I didn't hit my arm or anything, so there was no explanation for how it got there. It went away after a day or so and this week it came back in the same spot. I am a 20 year old woman, 5'4'', 115lbs. I was wondering what could be causing this bruising.