Two simple things to try...
Clean out the aerators on the sinks as they may be clogged with calcium deposits, it isprobably time to change out your anode rod in the water heater. Be sure to flush the tank thoroughly, I just stick a hose in the opening and let it blast away until the water runs clear.
Another possibility is if you have an outdoor shower check to see if the hot and cold faucets are shut tight. If they are ae open even a little they are mixing water into your system and could account for what you are seeing. Good luck.
:C