Yeah either you're trying to wash with the wrong kind of soap - apparently it's a terrible idea to use anything scented down there, I tend to disregard that because I'm too lazy to use two different soaps but clearly it's a problem for some people - or you're scrubbing too hard or something, orrr as other people have said you need to see a doctor.

The only times I've had that kind of pain has been when the skin's got irritated somehow and usually it goes back to normal within a day. If you've got persistent pain there to the point where it's making it hard to keep clean you should probably see someone.