The stress of losing her lifelong companion can cause various behavioural and/or medical issues to arise. Stress does manifest itself in many ways.
Cats that are de-clawed often have arthritis inflammation in the feet that were done. Because of her age this should be considered.
I would suggest you take her in for a Wellness Exam with special attention paid to her feet & full geriatric blood panel & urinalysis to make sure there are no underlying causes.
All cats over the age of 7 should have a full blood panel & urinaylsis done once per year, especially with cats 12 and over. If anything shows up on the tests it can be treated early & @ that point they should be retested every 6 months since cats are so good @ hiding any illnesses.
& keep us updated on how she's doing