It could be her age why she's snapping, dogs behavior changes when they get older .
My Westie is nearly 15 years old. When you stroke her she snaps at you. Why?
Being that age, maybe your dog is having some pain. Dogs can get arthritis and have other issues that cause them pain. A visit to your vet would be a good idea.
I would most probably say she might be in a bit of pain, as dogs get older they get sick sometimes as well, and as Angela suggest, arthritist can be an issue. Our dog does that as well, where he never did before.