My dog is wobbly and has weak back legs?

My dog is 6 years old and his back legs seemed to be weak so i took him to the vet and have him on a supplement for it, that was a few mths ago but tonight after his run his back legs started to go from under him and his paced around in circles, seemed wobbly too. Im really worried now as there is no vet around till monday. What could be wrong with him, all advice appreciated and if anyone has had this happen their dog before PLEASE let me know what you did. Also we got a pet rabbit last week could he have picked up something off him, dog has all hisvaccinationss???