May have to read the signals.
I agree that your dog may be a little stressed due to the (what may seem to her) sudden changes. She may have become used to reacting in a different way to her carer. You may have to return to the puppy training used when she was young (i.e. taking her out frequently and lots of praise for the good behaviour).
Her change in diet may have upset her system and it may take a while to return to normal. She may be remarking her territory if she seems to use a particular spot only. Dog urine is hard to completely remove, as dogs have extremely good noses and are able to still smell the spot - it may help to go over the area with some lavender oil if it is a hard floor surface.
Every now and then my westie 'forgets herself' and I return to putting her outside regularly. She even has to sleep outside all night sometimes! I become super-sensitive to where she is and what she is doing. Eventually she gets the drift. I always thoroughly clean the spot with carb soda and vinegar.