I always find that milk in the standard plastic jugs has an "off" smell to me after a few days of being open. My husband says he can't tell a difference and keeps drinking it. I think I have a more sensitive nose than he does, plus I hate the "plasticky" smell that the milk gets after sitting in the carton.

Have you checked your refridgerator temperature? It should be below 40 degrees F. If it's above that, your food will spoil faster. Also, the fridge doors are the warmest place, so if you store your milk in the door it will go bad faster (unless you have one of those nifty enclosed shelves that have cold air blown directly into it to keep milk cold).