That really does not sound like an STD at all. Herpes or otherwise. I think you are on the wrong track worrying about that.
Hello~It sounds like a type of fungus to me or irritation from possibly your thighs rubbing together. The nystatin might work, it is an anti-fungal product sold in either pill form, powder form, and I believe a cream. If that doesn't help, try using just plain A&D ointment in that area, or a diaper rash cream, possibly one of these might help to clear up the rash.