I Have A Bruise That Appeared On My Upper Arm

I have a bruise that appeared on my upper arm (without trauma) that is maroon in the center and dark beneath. There is a hard knot underneath it, about 1 inch around, that is not painful, but is tender and slightly warm to the touch. My arm is slightly swollen all around it as well. I don't know of having received any kind of bite, either, and see no evidence of one.