What Causes A Red Pimple Like Rash Near The Vagina Indicate?

Hi there, I am a 20 yr old female, 5ft 7, 140lbs. In the last week I have gotten a very painful small (about the 3cm in a diameter) bumpy rash at the top of my inner thigh just left to my vagina. It is red and pimple like in appearence. I have been sleeping with my boyfriend, and my boyfriend only for the past 7 months so I don't think its an STI. He is faithful too. It's really very sore though!