I'm with the folks suspecting solar heating of the hose and other plumbing. Over the years I've found it common for the water in the hose and plumbing to get pretty warm through the course of the day, especially if the hose is in the sun.
When camping on the nice warm sands of Saudi Arabia thanks to Uncle Sam, we were using water bladders sitting out in the sun and filled by tanker trucks to provide water for showers. You learned real quick that you either took a shower first thing in the am, or not at all unless it had recently been filled by the truck. The water would get uncomfortably hot over the course of the day.
