My skin is indented on my right upper buttocks, and lower back. At first, I thought it was a bruise, but it wasn't healing after 3 weeks or so. It was not until today that I noticed the large indent. Moreover, I thought I pulled my groin, or potentially pulled some hip muscle, because that it what it feels like. However, I am a long distance runner, and I haven't been bothered by it when running. What could this be?