Hi, I'm Dr STeve. This sounds like an anxiety issue from the presence of the new cat. Most cats will develop a relationship where they know who is boss and that the other cat is not a threat. Your older cat may not have come to that understanding yet. She likely feel threatened by the new cat as well as the change of her environment. Going from having you all to herself to having to share you and her house with a rival cat can be very difficult for some cats to cope with
There is a product called Feliway that is a pheromone spray that helps to calm a nervous cat and make them feel safe. I have had clients use this by putting the nervous cat in a familiar room with the spray and this can help them deal with the new cat.
By having a place to go where she feels safe, she may not be nervous or anxious all the time like she may be feeling now.
Unfortunately this doesn't work with all cats and it may just take more time with some cats. The only way to know is to try and see how she responds. Can you see my answer I am checking to see if the site is working
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