new aggressive behavior in my mature yorkie

Question
Hello!
My Yorkie just turned 3years old and has developed a new aggressive habit toward other dogs when we're out walking.  She's always been very socialized and a very sweet dog. This behavior is strangely new.  We live with one other (very mellow) small dog and a young cat.  My roommate has only had his cat for about four months and for some reason my Yorkie LOVES to play with this cat.  Could she have learned this from playing aggressively with the cat? And in any case, how can I get her to stop attacking other dogs?  It's embarrassing for me and dangerous for her.  The size of the dog doesn't even matter to her.  She sniffs them for a second and then goes nuts.

Thanks :)

Answer
Do you mind if I ask you a few questions that could help me answer better?
1. Did your Yorkie have off leash experience playing with lots of unfamiliar other dogs when she was a puppy?
2. Did the new behavior start only after the cat arrived in the household?
3. What do you do when your dog goes after other dogs, and does she only act that way when she is on her leash?