It was probably contaminated and it's basically buttermilk, which separates naturally and comes back together when shaken...not that I would taste it and find out.
You need a pressure
canner, not a pressure cooker.
1/2" headspace, 10 lbs. pressure for 25 minutes for quarts and 20 minutes for pints.
You'll know if the milk is spoiled by some bacteria because it smells like grits. If it's contaminated by something more insidious like botulism, it won't smell bad at all, but tasting it to find out might be the last thing you do. Dump it, buy or borrow a pressure canner, and can away. And get a copy of a food safety booklet (usually free, check here for sources:
http://www.uga.edu/nchfp/publications/publications_home.html) before you take on any more canning experiments.
Good luck and no worries. Everyone screws up somewhere...just ask the taste testers of my spicy pickled green grape tomatoes...
