Need help from cat experts

I have a five year-old orange tabby cat. She is a sweetheart but she has developed an annoying habit in the last month of doing ALL of her business in the middle of the night on my carpet, near the bathroom and my youngest son's door. Nothing worse than waking up to that! She is an indoor cat and has never done this before. We have a nine month old Sheltie whom she adores and plays with all of the time. I have always bought whatever cat litter was on sale - name brand only- so I have ruled out those two things as an issue. She is eating normally, her fur is shiny, eyes bright, etc. I just do not understand why she has developed this bad habit and how do I stop it?