Your dog might be having a condition called vestibular disease. For details, visit this link.
You would need to call the vet or emergency clinic immediately. Many things can cause that but is something you should get checked out immediately.
I have a 12 year old dog who is doing the exact same thing, He trembles, walks backwards, and pants. He has been to the vet at least 5 times over the last couple weeks. He stayed at the vet for a couple days then was released with antibiotics to treat an infection in the abdomen. The local vet here had us drive to a large city to have an ultra sound performed on his abdomen. The ultra sound showed an enlarged colon/large bowel but the vet has not been able to make a diagnosis. She is treating him with a laxative, two antibiotics, pain medication, and a steriod. I honestly believe he is in a lot of pain. We are going back to the vet here in an hour and I believe she will recommend putting him down. She doesn't believe he would survive surgery or she would order and MRI. I honestly believe he has a major abnormality in his abdomen which is causing pain. This is very hard on myself and the family. We have complete faith in the vet. She (Betsy Bialon) is by far the best vet I have ever met. She cares for her patients better than any human doctor I have seen in my lifetime and I have been through 8 surgeries including two at the Mayo Clinic located in Minnesota. Betsy is a very gifted person. However it's time for our puppy/angel to cross the rainbow bridge where he can be pain free and enjoy himself again. Goolge a poem named "Rainbow Bridge"
I would get your dog to a vet. It certainly does not sound like normal behavior. A vet can do tests and diagnose the problem, and prescribe treatment. Neither pet nor human ills can be diagnosed ovet the internet - you need to SEE a professional (vet for pet, doctor for human).
Maybe he ate something food which is not fresh.
I have heard there are neuralgic problems that walking backwards is one symptom that comes with this illness. Take your dog to a Vet
Maybe he saw a ghost or something you cannot see....