I have a weird skin pigment...thing...?

i have a skin problem...actually i have no idea what it is! D: on the sides of my hips(where my panties go), there's some weird stretch marks or weird skin pigment and it looks really embarrassing when i wear swimsuits. so i only wear the ones with shorts because i get embarrassed by it. i really don't know what it is...are they stretch marks? do they look like that because of my underwear? or is it just dirt or something? help me guys! idk wats wrong with me! i dont wanna go to a dermatologist lol :(