My cat is pooing outside of her litter tray

My 2-year-old cat has been pooing on the sofa for a few weeks now. Before she does it, she meows a lot. I’ve taken her to the vet and they couldn’t find anything wrong. The litter tray is cleaned every day, and she is still using it for both wee and poo, but about once a day she defaecates on the sofa. I really don’t know what to do anymore. There haven’t been any changes or specific events, and I haven’t changed the brand of litter since she was a kitten. She is also up to date with her worming treatment and is an indoor cat.