Anxiety in dogs is very common no matter how pampered a pooch you have. We have to work or go out so often they can get lonely & develop bad habbits arising out of separation anxiety. If it is this then buy a kong & fill it & give it to your dog when you leave the house. They will associate you leaving with them getting a kong. They will then be happy for you to leave them. Keep the kong only for this time. Use other treats as everday or anytime treats. Have chew toys & things to help occupy your dog while your out on ground level or in their bed for them. If you think it could be medical take her to a vet. She may have a skin mite infestation or fleas or a deficiency in her diet.
You need to figure out why she's chewing on her feet. Is it all 4 or just 1 or 2? Fleas and flea bites are often a reason dogs will chew on their feet. Make really sure she's flea-free and maybe apply one of the once-a-month topical treatments as an extra measure. If it's just 1 foot, gently and carefully inspect the foot to see if something has become caught there or if there's an irritation.
Kimkay and Schmeila are right. It could be anything from a mineral loss, something caught in the pads or an allergic reaction. Only way to check is by going to the vet. It could also be just a comforting habit. Vet will tell you.