Previously House Trained Dog Pooping Inside?

I have a 7-year old female Shiz Tzu who has started pooping on the floor, mainly in one room in particular. She has been to the vet and is healthy. I thought perhaps it was because she is bored so I have once again started walking her daily, but it's getting too hot for her at 90 degrees or more. Because of this going in the house (only pooping) she is once again being crated, how or what do I do?