Dogs shiver for a variety of reasons.
First and the most obvious to most people is the fact that they are indeed cold. Shivering contracts the muscles to stimulate heat. Since you have already addressed this issue--lets explore other possibilities.
The next two are medical and behavior issues.
Trembling and shivering are also indicators that a dog is in pain. If your dog is shivering on a constant basis you should rule out any medical conditions. You need to give her a complete body check to see if she has any sensitive or painful areas. Touch her whole body gently and carefully. Run your fingers down her back and legs gently, check under her tail. Check between her toes and check the pads of her feet. Look at her face, cup it in your hands and slide your hands down her chest. Check her eyes, ears, and mouth. Look on her belly and rub it gently. Even if she does not appear to be sensitive to your physical touch, there are still many metabolic problems that could cause pain. Make sure she is medically cleared by your veterinarian before assuming that the shivering is not medically based. (Especially if you notice anything else abnormal--not eating, listless, vomiting, diarrhea, limping, reluctant to run or jump, etc.)
Good signs that the shivering is behavior based is that it is NOT constant. She doesn't shake when she is eating, playing, interested in her walks or "gets to go for a ride," etc. Shivering can be a symptom of stress, separation anxiety, and or fear. See this site for more inf:
Dogs can also use shivering as an attention getting behavior. Remember, dogs will repeat a behavior that gets a positive response. That's the whole premise of positive rewarded--based dog training. So just like sitting politely (dog's behavior) brings forth a cookie (positive reinforcement), some dogs have found that shivering (dog's behavior) brings forth mom's concern in the form of attention (positive reinforcement.)
If you think this may be the problem you will need to determine what initiates this behavior and how you may be unconsciously rewarding the behavior. For example-- if she only starts to shiver when you start to leave the house and you notice the shivering and come back to pet her and "reassure her that its ok" then you are rewarding the shivering behavior. As long as the shivering is rewarded the behavior will continue! To extinguish this behavior you will need to start ignoring the shivering and reward her and pay attention to her when she is NOT shivering. You can also try distraction by playing games with her working and reinforcing her basic obedience skills. Remember NO positive reinforcement when she is shivering!
In short, make sure there are no medical reasons for her shivering and if not, then address the behavior issues. You also might find it helpful to enlist a good trainer to help you with her behavior issues if needed. Just make sure the trainer is a positive, reward based trainer!
I hope this helps!!