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Hi. My name is***** I'm sorry to hear about this situation with your cat. Can you take a picture of it and post it? How long has this sore been there? How old is this cat? What is his / her name?
Sorry for my delay. I got the picture. How long has that sore been present? It honestly looks like something cut her there. It doesn't look old and infected - like say an abscess would. I see some that look very similar where someone tries to cut a mat out of the fur and accidentally gets the skin.
I would be curious if she caught that area on something sharp and it tore the skin. That wound is big enough that a veterinarian needs to be involved with helping it heal. Ideally it would be seen right now by an ER clinic / your vet to have the best chance of cleaning it up and possibly suturing it. It may be saturable in the morning too. Some may get dried out and not be as ideal in that time frame. With her becoming more thin and her age, I would say that doing some blood work on her to screen for diabetes, hyperthyroidism and chronic renal disease is a good jumping in point for that issue. With her wound, it most likely is causing discomfort and when it is treated, I would hope she starts to act more herself.
Her environment itself has changed with the new people there and that can absolutely throw some cats off. As far as the wound, I would again be curious she tried to walk / under something and it caught her and caused this wound.