Help please. Could breast cancer show up on a cat scan of the heart or lungs?

Maybe. If large enough, it may show up on a ct of the chest, but probably not a ct of the heart (out of the field of view). But mammography is more sensitive than ct at detecting breast cancer. You should have routine screening exams, and a diagnostic mammo asap if you feel a lump.
Not the best. Way to try to see breast cancer. Maybe if the breast cancer was so large that one could feel a breast lump or breast mass, then it may be seen on ct scan. Mammogram & breast ultrasound are better. Dedicated breast MRI scan can also visualize well in some settings.