I would pop her in the bath for a minute, see if that calms it down. Ask her to show you the " soap" in case she has put something else more irritating. Then u need to check her vulva etc and see whether she has any rash or visible irritation. A soothing cream like nappy rash cream or mild steroid cream like sigmacort could help in shot term. If it's still painful and you can't see what's causing it a trip to the dr with a wee sample might be in order tomorow.