Cats can be very prone to stress and react in different ways. Loosing your dog will be the reason for this and hopefully it will eventually stop. In the mean time please provide a bigger cat litter tray and more of them in different places of the house. When she defecates pick it up and place it in the cat litter box so she can see it in there to try and inprint it in her mind that thats where its supposed to go.
The main way in combatting this is not to punish her as she will relate defecating or urinating as bad behaviour and she may start doing it in places where you can not see. Please be patient .
Everytime she misses the cat box move the litter tray to a different area. keep the litter constant and make sure she likes the litter( some cats do not like wood, stone etc..)
Keep a diary of times and places she goes along with faveroite litter box , all this will aid and help you in combatting this problem
Please bear in mind that some cats do this behaviour when they have a urinary infection , please examin the urine for blood , quantiy etcc.. If worrid take a sample to your vets (if you can get one by using non absorbant cat litter)
Hope i have helped