Welcome to WHL!
Many of us have been through the same thing with unfeeling docs. People can be sero-negative (it doesn't show up in the blood tests), but still have an AI disease, like Sjogren's or Lupus. The burning feeling in your toes could be Raynauds or Small Fiber Neuropathy, both are common with AI issues.
Keep trying until you find a doc who will listen.