My mother is being asked for a fit note?

Hello! I was wondering if anyone would be able to explain what's happening. A few weeks ago my mother received a letter stating that housing benefit is ending and she had until the 5th of January to move over to Universal Credit. She also receives Contribution Based ESA in the support group, which means she isn't being moved as Contribution isn't changing, and full PIP. We finished her identity check a few days ago and it's listed her first payment date as well as said in her commitments that all she has to do is check the account so all seemed okay. However just today it's stating that she has to submit a fit note? Is this just some sort of mistake? She's been out of work for a long time and been getting ESA for years at this point. It's really stressing her out and she's getting upset saying "It's about my other benefit they want to take my ESA away that's why they're asking if I'm fit" and I'm trying to find out what's going on to calm her down.