This Week I've Noticed That I Have Developed One Dark

This week I've noticed that I have developed one dark & super deep dent directly on/in the bone area, on the back side of my knees. I have lost weight if anything and have next to no cellulite. It's dark and deep and appeared out of nowhere. almost like it has sunk in overnight. What could this be? I experience little to no pain in my legs but I do live in the city and walk a lot. Thank you!