What a sad thing to have happen. Did they not give you any warning about it?? I really think you should have had a say in it. Maybe you should take a couple of days to get over the shock, then sit down with them and explain how hurt you are, not only because your cats are gone, but because they did not trust you enough to include you in on the decision. And to do it behind your back while you were at school is a very heartless thing to do. I am happy that they are adopted to a good home but I'm sure that does not ease the pain.

And Popcornbird is right. Your Mother will always be your Mother and no matter how you feel right now she will always be there for you no matter what. I speak from experience here. I really think communication here is the answer. Good luck.