Please help my carpet?

My cat recently started leaving poop skids on the carpet. This has never been a problem until her glands were expressed, and her food was changed. This all happened at the same time. Her previous food was Purina pro plan, now she gets natural balance green pea and duck dry food she gets the same moist food a few times a week. I have only seen her drag her a few time along time ago but none since the glands were expressed. The cat is 7 years old. Please help