This is a single indentation by your rear end? If this is what I think you are talking about, it's a sacral dimple about 3 to 8 percent of the population has it. So, not terribly uncommon. https://www.healthline.com/health/sacral-dimple#symptoms This article describes it. However, if this is new, please get it looked at as it also can be a pilonidal cyst which occurs later. (you are born with a sacral dimple).