What Does Growing Black And Blue Stain On Thigh Suggest?

i have a black stain on my thigh and i dont know what it is. ive had it for about 6 months now and didnt think it was a problem, it looks just like a black and blue but bigger. its black and purple ish and i dont know what it is, if you have any idea please tell me, also i dont have any medical problems and im 15 years old.