For one, an ARS-related rash wouldn't typically be just in one spot...sounds like a yeast infection. The groin would also not be a common place for an ARS rash...it would be more prevalent on the trunk. If you're worried, go see your doc!
ARS symptoms, when a person has them, usually present within 2-4 weeks of exposure, and can last 1-2 weeks.
Also, it would be RARE to experience just ONE ARS symptom. ARS stands for acute retroviral syndrome.