Why does my girlfriend get a rash on her thigh?

now this is very personal to me but i really want to find out the cause of this so my girlfriend doesnt have to go through the pain anymore. So after we have sex she gets this rash on the back of her thighs right under her butt, its like little red bumps. She said it hurts when she sits or touches the area, i think its from our legs rubbing together but im not sure, if any body can tell me how to prevent it that would b great thanks.