Frozen whole milk separates when thawed... why? Is it still good? Ick?

I froze 2 half gallons of whole milk 2 weeks ago... I thawed them yesterday & this morning noticed that while 1 of the half gallons was normal looking the other had separated. It looked GROSS. I shook the jug & it seemed to mix again. IDK, I have froze milk before & this never has happened. I always shake milk before I pour it (whether it has been frozen or is fresh from the store/fridge... I can't drink milk that has not been shaken). What causes the separation to occur & is it still good once shaken VERY well?