If you mean that the stainless steel pump casing is getting hot, as opposed to the motor casing, then there is a problem with the pump controller - whatever that might be, pressure switch, electronic Presscontrol type, or whatever. The cause of the pump casing heating up is sdue to the pump running for an extended period with no water passing through it. The result is the water becoming heated by agitation. This situation can arise if, for some reason the pump finds itself unable to achieve its intended cut out pressure - usual causes are blocked suction, air ingress in suction pipe or debris in the pump reducing performance.