Thank you for your patience. As you can probably appreciate, there are a number of reasons why Zooey might be scratching excessively all of a sudden, and we have to consider the most common causes first and foremost. External parasites is the most common cause of scratching in the dog, however given your other dog is ok, we would more specifically be concerned about fleas, and whether Zooey has a possible flea allergy. It can be very hard to find evidence of fleas if there is only a low burden of them, however it only takes one flea to bite a dog with a flea allergy for there to be intense itching for days. As such, you are best to treat both dogs for fleas first thing tomorrow using an appropriate flea treatment for their weights. Use a reliable product such as Advantage, Advantage multi, Nexguard or Bravecto - all of these are very good.
We also have to consider an environmental allergy, and given this initially started at home, it could be from anything such as dust mites, to mold/mildew, to pollens and grasses. This would also explain whey Zooey is affected, but not your other dog. If Zooey is particulary itchy over her back and around her tail base/rear end, then we also have to consider whether she could have full or impacted anal glands which can cause this sudden intense irritation back there. Also be sure to keep both dogs up to date with their deworming treatments, as intestinal worms can cause irritation around the anus and tail base as well. Drontal and Milbemax are two very reliable all-wormers you could get for them if they aren't up to date.
Other causes include a food allergy (although her food hasn't changed), an allergic reaction to something (such as bug/bee bite or sting), or itching secondary to an internal issue. Given she is an older girl, if she hasn't had a full senior check up recently, then I would definitely recommend this. It would be very rare for this scratching to be due to another internal organ cause, but we can sometimes see that with a liver issue in dogs and cats.
For now, you need to do what you can to stop your girl from hurting herself. If she is licking and scratching excessively, I would definitely recommend that you get Zooey and e-collar/cone to stop this. You can also give her a bath with a gentle dog shampoo, and an oatmeal based canine shampoo is ideal for this scenario. As above - get both dogs up to date with flea and worming treatments asap (tomorrow as possible). For Zooey in the meantime, you may want to try her with a little Benadryl - the typical dose is 1mg per pound of body weight up to twice daily. You can read more about the use of Benadryl in dogs online here: http://www.petplace.com/article/drug-library/library/over-the-counter/diphenhydramine-benadryl .
If she is scratching herself raw, or if there is no improvement tomorrow, then you will need to find a nearby vet in the city you have traveled to this weekend, for a full check up, senior blood test and skin diagnostics. All the best with Zooey and I hope her itchiness settles soon. If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
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