Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that you have been waiting a while for an answer but your question isn't an easy one to answer without seeing your dog, and knowing her medical history as well as her behavior and the dogs that she is growling and snapping at that are trying to sniff her.
You mention that Dot is nervous.
Is this something new for her or has her anxiety increased significantly recently?
Has something changed in her home environment that is leading to her being more anxious? (new baby, new pets, change in work schedule ect)
How is she feeling? Does she have any chronic health issues? Dogs that are in pain or are feeling poorly may behave more aggressively because they are in pain or not up to socialization. Dogs that aren't feeling well may feel especially vulnerable. They are concerned that they won't be able to defend themselves.
Dogs that are already anxious react poorly to yelling and being harshly corrected. That only increases their anxiety and makes them associate other dogs with you being upset and her getting yelled at.
My first recommendation would be a thorough physical to look for signs of pain and some blood tests to make sure that she is healthy.
If she is healthy then reducing anxiety and improving her confidence may help with her social behavior.
You can try supplement products to reduce anxiety/fear. None of us learn well when we are afraid or nervous. One examples is DAP (dog appeasing pheromone) which is a synthetic version of a calming pheromone nursing bitches produce. It comes in a spray as well as impregnated collars. Another option is a calming homeopathic remedy called Bach's Rescue Remedy. It can be used along with DAP. See this link for further information: http://www.bachflower.com/Pets.htm
If that isn't enough your veterinarian can prescribe anti-anxiety medications for her. If we can get her relaxed enough to interact with other dogs in a calm manner perhaps she can learn to enjoy herself rather than feel threatened.
She is fairly young and they likely won't need to be forever, just until she learns to trust that she doesn't need to be aggressive with other dogs.
We need to focus on building her confidence in general. A program that may help with that is called "nothing in life is free". This gives her confidence in your fairness and her ability to please you and where she fits in your home as well as confidence in herself and how she should behave. Here is a link that better explains the program: http://k9deb.com/nilif.htm
Obedience class is a wonderful thing to build confidence and practice being around other dogs in safe, nonthreatening environment. She learns what pleases you, gets rewarded for it and learns to trust you and that if she does what you'd like she is safe and won't get into trouble. She also learns to do this in front of other dogs and people in a nonthreatening environment (because the other dogs are focused on learning training, and not on her, so she may feel less intimidated) and may make some new friends which will also increase her confidence.
There is no quick way to helpful fearfully aggressive dogs. You may make much faster progress if you work with a trainer.
Let me know if you have any further questions