Beazly:
I would guess that either she got a bit car sick that one day or something managed to spook her unbeknownst to you. This can cause them to associate that feeling with where they were at the time. So, if she twisted her paw, a big truck spooked her, or maybe a quick stop startled her, she would not want to get into the vehicle to go. I would have the vet give her a quick go over just to make sure she does not have an injury going on, just to be on the safe side. Once that is done, you can start retraining her to enjoy the riding. Go for short rides to enjoyable places like the pet store that allows dogs to pick out a treat, to a fast food place to grab a burger patty ;) If you notice any excessive salivating talk to your vet about the use of motion sickness medication to help get her over it :) Each time you can go longer and longer until you are back to where she used to be at her enjoyable one hour ride to and from work :) Good luck to you and you girl :)
breeder/healthcare specialist
15+ years animal health care Hobby breeder of Olde English Mastiffs and owned by an English Bulldog