Change is very difficult for many cats, and a new kitten can cause a lot of stress. Just because she seems to have accepted Sophie, it may be bothering her more than you've realized.
Stress can make a cat sick, and any change in behavior can also indicate sickness or pain. The first thing to do is to take Millie to the vet for a complete check up. Have a urinalysis done, she may be developing a urinary tract infection (a common illness brought on by stress)
Have you had a chance to observe her litter box habits to see if there is any change in her peeing or pooping?
Declawing: please read this first
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