Hi and welcome to HCM.
It doesnt sound like STI but this must be ruled out of course. Usually this is clogged gland or bartholini gland and inflammation of it. This is commonly found in young women. If there is no Vaginal discharge
or pain while urinating then it shouldnt be STD
bacterial. But some viruses such as herpes
can manifest on such way. I suggest to wait and see will it progress or you notice pus formation.If yes, you should also do vaginal swab to see the exact microbe which causes it and then it can be treated on right way.Till that keep it clean and dry, and avoid syntetic clothes that provoke sweating. Wish you good health.