I have a 9 year old shelty,and I have just started to notice she is going really thin at the back end!do I need to worry?

There may be dietary issues with a dog that old. There may be arthritis issues. There may be medical (internal) issues. Look at this chart and see if there's a cause for concern. If so, make an appointment with the vet.

dog weight chart - The Purina Body Condition System