First, I would stop taking her around other dogs until you've had a professional assess the situation and guide you on how to modify her behavior. The last thing you want to do is make every experience with other dogs a stressful and negative thing. That only makes things worse.
One thing to note is that most aggression in companion animals is fear based, so the fact that she is timid is an indicator. You want to make sure that you're not rewarding her fearful behavior, and that you're not pushing her too far. Take note when she starts getting timid and try to manage those situations carefully.
You can talk to your vet about a behaviorist. He/she might know of someone in the area. You're looking for a certified behavior counselor, not a trainer.
Best of luck!
Julie
