Hi,
It could be a variety of things, you would have to be diagnosed by a doctor in person to figure out what is causing the appearance of a 'rash'. Make an appointment with either your GP or gynecologist (you can also visit a health clinic) for a pelvic examination, in order to determine what it is, and how to get treatment for it. Your boyfriend also needs to be examined and seek treatment, at the same time.
Take Care