It would be quite normal for a Hot Water pump to get hot, for obvious reasons, but assuming the other is for cold, then it should only warm slightly under load.
For the cold ump to overheat, I can only think that the immersion cylinder has far too much hot water, and this has expanded into the roof tank (from where I'm assuming the cold pump is fed.)
Turning on the heating has maybe allowed the excess heat in the water to dissipate.
This is all assumption, since I don't know the type of system you have. Possibly boiler thermostat gone faulty (on the HW side)