Have a sunken spot on my leg. What could it be?

See a doctor. If you never saw it before, see a dermatologist. Good luck.
Few thoughts... A few things that come to mind include: atrophy of muscle in the area, a reaction from a steroid injection or topical steroid, fibrosis of the area maybe after some a trauma... Or it can just be a normal variation of your body! if it's developed over a period of weeks to months, it's something to have checked by your doc - not urgently, but sometime in the near future. Best of luck.