Sudden aggression

I have a wonderful rescue adopted in July 2011 and probably just turning 1 year now.  I have 4 cats that she has gotten along with and one that she played with until 3 weeks ago.  I had to go for surgery and she was boarded for 18 days.  Just brought her home and she has aggressively attacked her one playmate 4 times.  I now have a muzzle on her because it happens so fast without warning.  Tried separating by gate, which the cat jumps over and then Lilly (dog) goes for her.  If I command her to stop, she has a couple of times but once she is on him, there is no stopping her until she is physically removed.  I thought working with a leash and the cat might be a good start but I am hoping for some insight and I don't want to endanger the cat for even a moment.  I have crated her as well as locking the cats up and allowing her playtime but am willing to do whatever it takes.  She is a truly sweet dog otherwise and I don't want to unintentionally train her incorrectly.  The boarding was in a home with other dogs (mostly smaller) and no cats.  She was not crated and the owners came highly recommended, posted videos of her playing and happy. Thank you,