There can be long term problems associated with declawing, a trip to the vet to rule out anything physical is always the best place to start. (My mother's cat had growths- the vet described as almost like the claw trying to grow back- 8 years after being declawed and they were bad enough that she started limping.)
Also it can be anxiety from the change in the house, the loss of her companion. You can try rescue remedy or something like feliway or they even have anxiety meds for pets as well if you rule out the physical possibilities. It is also possible for them to develop an allergy to food or something in their environment (my 'mower' is allergic to fleas and corn).
Best of luck in figuring it out!
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