My water is discolored after a few days

My camper is set up more or less permanantly in one location. It is connected to a private well that sources water 600 feet down. It is unoccupied one to three weeks at a time and the water is turned off when I'm not there. I come back, turn the water on inside and it is PINK! It's not coming from the well - outside faucet doesn't do this and camper waterclears up after running it a couple of minutes. So, it must be something inside the trailer piping that is leaching into the water. I don't drink the pink water but after I run it a couple of minutes to clear it up, it tastes just fine. Any ideas of the source or how to clean it? Thanks