What Causes Dark Mark On Thighs Near Knee?

Hi Doc, I am getting dark mark on my thighs near knee in triangle shape....it looks like tan marks. I don t feel itchy and no burning sensation...its just mark I am getting in my both my thighs in same place...I have not expose to sun after last winter in shorts....so I don t think it will be tan mark...Please advise why suddenly I am getting these marks...and what should I do?