Older dog suddenly doesn't like one cat

We have had an 11/12 year old lab Irish setter mix for the past 9 years. She was a rescue so exact age is hard to tell.

We've had cats for the last 5 years, a tiny cat she didn't want to like at first but eventually accepted as her best friend, and that cats two daughters about a year younger.

In the last few months, our dog had decided she really does not like one of the cats. She doesn't go after it but if it gets close or tries to rub on her like the other cats do, she starts snarling and shows her teeth.

There hasn't been a bite or attack yet but we're really worried as our dog gets older and gets crankier one day she might hurt the cat. We try to keep them separated as a precaution but we both work a lot so aren't home for long stretches. Any help would be appreciated.