What your dog is reacting to
A vet clinic is a hard place for a dog. It smells of other animals and cleaning products. It is full of strangers who touch, poke and restrain. Many dogs come in already carrying stress from the car ride. And nearly every visit ends the same way, with something unpleasant.
Dogs learn by association. If the clinic always means a needle, the dog learns to fear the lobby. A dog who bites or struggles is not bad. They are scared and trying to protect themselves. Look for the signs early: lip licking, yawning, whale eye, trembling, refusing treats, and panting. See signs of stress in dogs.
Always tell the clinic that your dog is nervous. Good staff want to know, and many clinics have low-stress handling practices. Pain can also make a dog snap during exams. Ask your vet to check for pain if your dog's behavior has changed.
Try to track which part is hardest. Some dogs are fine until the scale. Others fall apart at the sound of a certain door, or the moment a stranger grabs a leg. Once you know the worst moment, you can practice that part at home and ask the clinic to adjust it.