Treato found 16 discussions about Cold Symptoms and Cats Allergy on the web.
Best Answer: When the toilet is flushed, the toilet is robbing pressure from the shower, which is usually close to the toilet. As the COLD water pressure goes down from the toilet flush, the HOT water pressure is higher, therefore the water gets hotter as the COLD pressure goes down.
Check if coming from hot or cold water only - that could narrow down the cause. (Remember toilet may receive both hot and cold water if it has a tempering mixing valve to prevent condensation on the tank - incoming water will be warm, not cold, if you do).