If your milk is just old, it is probably not bad. This is called yogurt and millions eat it every day. You'll be fine. In fact, you should go all out and chug the entire carton, you might get to see some funky stuff you normally wouldn't, like a girlfriend. Milk really can't be bad because otherwise it'd be bad for our mom's breasts and they'd be dead therefore no women, so we wouldn't exist.
However, it is possible that if your milk tasted bad bad it was provided to you by a cow who died whilst being milked. Its vengeful spirit will continue to hex all dairy you drink. Stick to healthier beverages like water and red bull. You'll pee it back out, but it's going to be hard. Eventually you'll have the feeling of needing to take a big piss and all that sludge will just come out. The proper procedure is to immediately remove all traces of milk from your system. The only way to be sure is to surgically remove your entire digestive system. This is difficult to accomplish on your own, so I would recommend getting the help of a trusted friend. If it's bad enough, you could die.