Hi. No, that doesn't sound like any std I can think of. Are you sexually active? That's a key part of getting an std. So, some girls have hyper pigmentation. If your skin in the bikini area produces too much or more melanin, this can be the result. That's not a serious thing but a nuisance. Can you tell your mom about it? Again, that's not the sign of an std. https://www.healthline.com/health/dark-inner-thighs