There are various skin conditions that can cause a brown (ish) rash. There is a fungal rash called tinea versacolor that is like a yeast infection on the skin. That's the best way I can describe it. There is another that is bacterial called erythasma. This happens sometimes due to hygiene issues (if you don't clean that area), humidity so it gets moist that can happen inside your clothes, if you are overweight or if you have diabetes. There are other things that will cause a brownish rash in this area as well. A dermatologist can diagnose it and treat it. If you have a fever at all, see a doctor immediately. Let us know the result, okay?
First, it's important to not scratch the area- I know that is tough when it itches, but work on not scratching- that will only make it worse. Next, it's important to get to the bottom of why your skin has changed in color and is itching all the sudden- that will take a doctors exam- where they will need to gather more symptoms and medical history. Make an appointment with your doctor and get checked out.