Hello and welcome to the forum. We are sorry, however, that you have to ask this question as cancer is very stressful within a family. Your mum's findings seem to indicate something new is being seen on her imaging, correct? Her doctors hopefully will fully answer her questions but appear to be taking appropriate action to investigate further. The new findings could be benign and hopefully are but they want to make sure. And this will help them adjust her treatment going forward. Please let us know what her next step is and what they find. All the best to you and your mum.
Different modalities have different strengths and weaknesses. For example, CT chest is good for lungs, not so good for breasts, which often appear dense and heterogeneous. If there were breast nodules detected incidentally on CT chest, it just means they need to be worked up further on the appropriate modality, such as mammography, breast ultrasound, and/or PET/CT. Given your mother's history of breast cancer, it sounds like the doctors are taking appropriate next steps to determine if the findings on CT are benign (e.g., fibroadenomas) or malignant (e.g., residual/recurrent breast cancer).