I debated for awhile on whether to post my doggy experience similar to your’s but decided to even tho it’s really sad.
My basset started limping one day out of the blue. We gave her pain meds thinking it would eventually heal. Well after several weeks there was no improvement so took her to the vet. Xrays showed nothing so the vet suspected the worse. He was right. A biopsy came back positive for bone cancer.
I sincerely hope that is NOT what’s ailing your bestest buddy, but it is a possibility.
Hildy, did they check for Valley Fever, it is an issue where we live and sudden lameness can be a symptom. But I would check the paw very carefully to see if a sticker is imbeded and festered up (have had that issue)... if it is you can soak it in warm black tea to pull out infection and then apply fresh aloe vera. I never give the anti-inflammatory meds the vets prescribe, too hard on the liver.. much better to give bosewellia or tumerick. Cosaqiuin for dogs is good, too.
“Bone cancer”
That’s the other thing that came to mind, but didn’t want to raise the issue because it’s scary.
A deep bone bruise is another possibility. At this duration, it’s likely NOT a soft tissue injury. So we’re talking bone or systemic (arthitis, liver, diet, etc.).
As another poster said, it’s probably time to get a new vet. Or at least a second opinion...
A friend had a similar experience with her dog - thought it was a leg injury that kept being re-injured & it was treated as such. After long months, it was finally diagnosed as bone cancer. Vets are like people docs - you have to be proactive, do your homework, ask a lot of questions & advocate for your pet.