What Causes Pain And Indentation In The Muscle Of Upper Buttock?

I have pain and an indentation in the muscle of my upper buttox, almost thigh area. I did not injure the area in any way. The pain started about a week ago and just felt like I pulled a muscle, but now there is a very noticeable indentation. I am an active 33 year old woman, I have had 3 children. I am 5'10" and weigh 150lbs. I have no health issues.