i have to run my water for a few seconds, otherwise the water is cloudy and stinky.

my plumber has been here moving pipes and stuff, but not in a couple of days (i haven't run a lot of water from my new faucet either).

could it be the old pipes, as i am sure he will claim it is, or could it be something else? like, my fear, water sitting in the trap coming back up? is that even possible?