In addition to something unknown, a couple of things come to mind. When a water softener screen fails, the resin beads can get out of the softener and into the water system. Every water softener is filled with small zeolite resin beads. These beads are designed to remove calcium and minerals from the water and replace them with sodium or potassium ions. When the filter that holds in the beads develops a hole the resin beads can flow through the water supply system.
Inside every water heater there is a plastic tube that takes the inlet cold water and runs it down through the hot water to the bottom of the water heater, so the cold water doesn't mix with the hot water and prevent you from getting hot water. Unfortunately, some types of water actually dissolve the plastic and cause it to flow out into the water system. If you are now getting lukewarm water instead of hot water, that is your problem.