I need advice on my dog Sadie ASAP!!?

I have had my dog sadie since she was 3 months old, she was dumped out of a car on the streets. We think she is a lab/pit/germansheperd mix but we're not really sure. She has been socialized since we got her and has never really had any problems until lately. She is about 3 now and yesterday at the park she was on a leash and a little maltese was not. The other dog was not showing any signs of a problem and my dog was wagging her tail as she smelled the other dog. the other dog was just smelling the gr and then all of a sudden my dog attacked, she picked her up and shook her like she was trying to kill her. luckily we pulled her off and the maltese was ok. She was bleeding but after taking her to the vet she is ok, no stitches needed. I am not sure what to do since sadie has never shown any aggression before. Does anyone have good advice or know if a good dog trainer in san luis obispo?