I Have A Place On My Arm...several Weeks Ago O

I have a place on my arm...several weeks ago o woke up with a deep red bruise about the size of a dime. I figured I did something in my sleep. It went away I. The normal time frame. About a week. Yesterday I woke up and noticed a slight discoloration I. The same place. By afternoon it was a deep red bruise. It is on my left bicep. I do not lift weights. Should I be concerned? What causes things like this. I am sure it was not impact related.