Elderly Dog Pooping Inside?

I have a 17 year old Chow Chow/sheltie. She's started pooping in the house sometimes. She is stiff when walking around, joint supplements in her food have helped that and she is no longer in need of anti-inflammatories. She doesn't squat great, but has some better days than others. It isn't during bad weather. She doesn't pee inside. It isn't diarrhea or loose stool. Sometimes it's really hard, other times it is normal.

We have a doggie door for our dogs that is open night and day, and she uses it all the time. She knows that it is wrong. She will go outside 5 times within an hour, walking around the house, in/out over and over, and then poop on the floor next to us at the computer. She'll be half squatting with her tail up and just start going as she's walking around the house. Sometimes it is on carpet, sometimes it is on the linoleum.

I've tried making her go outside every time she wakes up. She's a happy, loving dog and isn't in pain other than a bit stiff when getting up (so am I, but you're not putting me down either.) She still plays and trots around a lot.

I mentioned it to the vet the last couple of times she was there and they just said "she's old and is starting to lose her ability to know when she needs to go". She doesn't pee inside though. The only other time she has accidents is when she goes to the vet. It does scare the poop right out of her and that is just nerves.

We have one other dog that she's lived with for 10 years. They have never fought over food, toys, or territory. They're both spayed.

Any helpful suggestions are appreciated.

By Cassee