How do you appoach? Do you look her in the eyes? That is a sign of aggression to a dog. She is intimidated by you. Your ALPHA dog to her and this is how she would act in the pack.
Also her rolling over and exposing her belly - Sign of submission. This isn't good but can be overcome to a good degree. My dog Shaggy couldn't even be touched without rolling over onto his back!!! He was so shy and timid.
There is a solution. It will be kind of tough but it needs to be done. It isn't an overnight thing as well. It is an ongoing training process and the results just get better.
First, DO NOT approach looking into the dogs eyes. Put some food or a toy out in front of the dog. You want it's head down to pet. LET the Dog SMELL your hand as well. If the dog flinches while attempting to touch put your hand out for it to smell. This gets the dog familiar with your smell and knows who you are. ALWAYS have a dog smell you BEFORE petting.
This step will take some time believe it or not. Give this method a few weeks or days until you both feel comfortable to move on.
Second, make sure the dog KNOWS the "Pecking Order" of your house. You are ALPHA dog, the next dominant dog if you have a second one is your BETA dog. However, treat the BETA dog as an ALPHA dog at all times. Give it treats first, feed first, anything first. This establishes the pecking order and stops the fighting between animals. They growl at eachother to establish dominance. This dog in training is NEVER going to be the dominate dog and it is dangerous for you try to change that.
Third, this is the hardest thing for me... EVERY TIME the dog rolls over on it's belly TURN IT OVER!!! I had to Grab BOTH Tail and HEAD and FLIP Shaggy onto ALL 4's. He tried to nip at me a few times but DO NOT Rub the dog UNLESS the dog is on it's feet!
This take lots of time and patience. This builds the dog's confidence up and makes it less submissive.
My dog Shaggy NOW jumps up on me and LOVES to be played with!!! It is remarkable the change. NO one recognized him based on his behavior. ALL the kids volunteer to walk him now that he is approachable and loveable. He even got trained to take Doggy Hugs!!! He jumps on me JUST to get a HUG!!!
It takes some effort but it is worth it. I have 4 dogs so the dominance thing is important in my household. They ALL share the SAME dish of Food!!!