What Causes Blister-like Bump Near The Buttocks After A Long Walk?

Hello. I am 18 years old. Recently I walked a very long distance, and I sweat profusely. So, walking for over an hour causes me to sweat massively in all areas. That day I wore a thong, and tight jean. I had nothing on my vaginal area, now I do. I the end of my vagina, near my butt (where the thong leads up to my butt), I have a blister-like bump on each side. They are tender, more bumps that blisters in my opinions. Is it possible it could be from chaffing and sweat against the thong? Or is it an STD?