Recently has hot water coming out from the cold pipe. Why is that?

This is usually caused by a cross connect between hot and cold. As a rule from work that was just done on your system or in a mixing valve such as a tub/ shower valve nor any single handled valve. Tell me more. If this just on one faucet or throughout the entire house? Have you had any work done on the system reascently? Go feel the cold water line feeding the hot water heater. Convection will carry the heat on up the cold line for a way. Does the warm water extend past the first branch off the cold water supply? Let me know, Tom