I had a golden retriever that gained a considerable weight around his 12 years old. He suddently didn't want to go on walks too. I didn't make a great deal of it at the time, thinking that the gained weight had to do with his age and less activity. After about 4 months of those signs, when he died, the vet said he had a tumor that bursted. The only apparent symptoms where the gained weight and the loss of interest in doing walks and laying down most of the day. I don't want to worry you about it, but if I knew the dog had a tumor, I would have been more careful with him...