What Causes A Tender Raised Bruise On The Arm?

I have a raised bruise on my forearm that is swollen and very tender. It is red in the middle. I can not recall hitting my arm on anything. I normally wouldn't think much about it but I had the exact same bruise and knot on the same arm. exact same spot about a month ago? I've tried to think what I could be doing to cause these bruises but can't come up with anything. I thought if it happens again I would go to the doctor but I wondered what you thought