Aussie suddenly refuses to walk

Question
Our Australian shepherd, Brandy, is one-year-old. We live in Florida so I'm out with her by 7:30 a.m. for about a 45 minute walk and then about 8 PM for about a 20 minute walk. A few days ago she started refusing to walk outside and would plop herself down like a lump. The first time it happened I thought she was hurt. It has progressed to where she will get out the door and plop down on the driveway and refuse to move forward. She wants to go back into the house. I have taken her to a park that has a dirt path thinking perhaps the pavement was too hot. The first time she was fine. The next day she plopped herself down. A man who was jogging suggested that it was just too hot for her and I should take her home. She loves going for a ride. I don't see any other behavioral changes. She is still obedient with the commands she knows. We have a small fenced-in yard and I do go out every few hours with her (and our two senior German shepherds) to throw a frisbee or ball. I'm concerned that something happened to make her fearful and that if I don't get this under control she will eventually have behavioral issues owing to lack of exercise and a release for her energy. If this was her first experience with a collar/lead I'd understand it, but I have over several months worked on increasing the amount of time/distance we are walking and getting her acclimated to the heat. Any suggestions?

Answer
Perhaps the passerby was right, that it is too hot for her, but any time a dog exhibits a sudden change in behavior, I suggest a trip to the vet to rule out anything medical.  Some dogs do go through additional fear periods, even at this age, but with Aussies, you also would want to be certain that she is not suffering from hypothyroidism, which runs in this breed, and can cause intolerance to exercise.  A full thyroid panel can rule that out, and if she does have it, the cost of the medication is fairly low.  If the issue is fear, then you could speak to a trainer about helping you master some counterconditioning techniques, but do get the vet visit out of the way first for safety's sake.  Good luck!