Suggest Remedy For Brown Spot And Back Of Left Thigh

Hi! Im 15 years old, Im 5'3 and about 130 pds. About a year ago I noticed a a brown spot on the back of my left thigh. I didnt know if it meant anything but just recently i've gotten one on my stomach, and im starting to wonder what it might be. Can you give me any feedback on what you think it might be? Thank Youu