My my mum had a mammogram a few months ago and they gave her the all clear about it, but a CT scan showed a nodule on her left breast and she has an appointment on Monday about it.

I’m a bit confused as my mum went for the mammogram and check her all out and gave her the all clear. Afterwards she had ct scan with contrast because she’s been getting short of breath and had a abnormal ecg result and they cleared her heart and lungs but they said they found a nodule in her left breast. Obviously thinking the worst my mum was very upset. The NHS got her booked in on a 2 week appointment to go and see someone about it. She told me something about a biopsy? I’m just confused as to how a mammogram would give the all clear yet a ct scan shows a nodule? What does this means and what will happen now? I’m in the UK, any advice is really appreciated thank you!