Does the rash itch? Sometimes urticaria can present like this, particulaly a subtype called 'cholinergic urticaria', the mainstay of mamangement being antihistamines. That, however, is a rare condition and it may just be that the water is too warm or that you are sensitive to a particular soap. Do you get the same symptoms when you become warm, say from exercise or by being in a hot room?
edit: Cholinergic Urticaria is usually intensively itchy.