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Are you getting warm water in the toilet too? do you have a recirculating pump on the hot water heater?
Ok, it sounds like you have a cross over (hot water crossing over into the cold piping). A re-circ pump is probably the most likely culprit with an issue this bad. Another possibility is a mixing valve that's nto working right or a check valve that's not working. Sometimes mixers are installed to mix a little hot with the cold going to the toilet so the toilet doesn't sweat. If you have one of these that's another likely culprit. Bot***** *****ne here is you're probably going to need a plumber to fix this. I can continue trying to help with some troubleshooting but you don't seem to be very familiar with these systems and it's doubtful you could fix this even if we can pinpoint the problem.