What Does A Circular Indentation In Between Buttocks Indicate?

Hi...I literally JUST discovered a circular indentation in between my buttocks. It's close to the top...probably an inch or less away from where the buttocks meet. It's not very deep, but most of the tip of my index finger is able to fit inside the indent. I'm really worried because it's never been there before. I will say that the same area has been swollen and inflamed before. When this happened, it hurt to touch.

I'm 20 years old, female and I've only ever had two sexual partners in my life. One female (the sex was unprotected), and one male (we've never had unprotected sex).