At this point, I'm going to vote that anyone who actually knows what happened not tell you. I can't imagine there will be an answer that will result in you saying 'Man, I'm glad I put that in my mouth!'
It's definately spoiled milk. It may look and taste normal if it's only like 2 or 3 days old, but I've had this happen once before. Except I was drinking coffee, but there was the exact same stuff after I had drank all the coffee and I was like wtf is that stuff :x