My dog Annabelle is having problems with her back legs, and I have no idea why. Most the time she is fine, but sometimes they collaspe under her. I took her to the vet, and was told that it was just part of ageing. She turned 16 years old in May 2011. She doesn't seem to be in pain, and doesn't have a problem when I take her out walking. I don't have the money to take her to another vet. Any suggestions on what I can do for her?