Sour Milk?

If you are breastfeeding, and your pumped milk tastes bad (can range from soapy to rancid) and your nursling will not drink it, even when you've followed good storage procedures, the problem is likely excess lipase in your milk. This is not a problem when baby nurses "from the tap" or with freshly pumped milk, but after a short time at room temperature, or in the fridge or freezer, the lipase starts breaking down the fat in your expressed breast milk (EBM) and it changes the flavor. If your baby will drink it, it's not a problem, but if your baby refuses the milk, you can scald the milk stop this from happening.

More info on excess lipase from Kellymom:
http://www.kellymom.com/bf/pumping/lipase-expressedmilk.html

I dealt with excess lipase on the rare occasions that I pumped milk. (The Little Missy would never drink EBM, no matter how fresh or how many times I tried.) Here's what I found about how to scald the milk easily, and it worked well for me when I did it. I pumped into a standard bottle, then put the bottle in a Munchkin bottle warmer and use a little extra water than the instructions said so that it would get extra hot (I used almost 1/4 cup) . I bought a digital meat thermometer that I used to make sure the milk would get up to 180 degrees (or 165 degrees for a full 15 seconds), which wasn't difficult. Then I put the bottle in a cup of ice water to cool it down quickly. Just don't do this with glass bottles or they can break. (I used the BPA free "breastmilk storage bottles" from Evenflo, and I know the Medela ones are BPA free too.)

Here is a very long thread on the LaLeche League forums about various ways of dealing with excess lipase. (This is where I found the tip about using a bottle warmer.)
http://forums.llli.org/showthread.php?t=297