All 4 of mine dream- feet flying, growling, with "bif bif bif" noises- .. Its so funny to hear this low growl and barking out of dogs that do not do this otherwise when they awake.
I have noticed it is more triggered if they saw a wild animal earlier in the day. Or especially the days following lure coursing events etc. And the girls do it more than the male dogs.
You might want to take a look if she is seeing other dogs during the day off your property. Then the worry shows up in her dreams. I know the first time Hottie saw a bear on the deck, that night he bolted up out a sound sleep and a big " WOOF!"..He also can't really move that fast to get up if he were awake. It must have been a really intense dream.