What Causes Red And Purple Color Bruise Near The Top Of Buttock Crack?

I am a healthy 39 year old male and I just found a mysterious half-dollar size bruise, red and purple in color, near the top of my butt crack, tail bone area. I did not injure myself, the area is not sore or tender, and it has no texture. I have no known disorders other than acid reflux for which I take omeprazole and renitidine. Any suggestions?