This reaction can be caused by a number of things:
The cap or foil was placed over the area where the stripper was applied Ė this may have acted as an insulator, stopping heat from the scalp from escaping and causing the product to become hot to the touch. ...More >
Some home colour can contain metallics dyes and can react with cleansing products causing heat as a bi-product of this chemical reaction
Metals in the water can build up in hair and can have the same effect.
Either way, if a product becomes hot to the touch I would remove it from the hair ASAP as this can cause lasting damage to the hair or scalp.