Benign Ovarian Cysts: The presence of stable and new septated cysts without solid components, combined with relatively low and stable CA125 levels, suggests a benign nature. The fact that a CT scan with contrast done 6 months ago was clear further supports this diagnosis.

Simple Ovarian Cysts: These are common in postmenopausal women and can be septated. The absence of solid components and the stable nature of the cysts, along with normal CA125 levels, make this a plausible diagnosis.
Hemorrhagic Cysts: Although less likely given the stability and description of the cysts, hemorrhagic cysts can present with septations and can occur in postmenopausal women, especially if there's a history of hormone replacement therapy.

Ovarian Cancer: Although the presentation and CA125 levels are not highly suggestive, ovarian cancer must be considered in any postmenopausal woman with ovarian masses. The appearance of a new cyst, even without solid components, warrants careful monitoring and possibly further investigation to rule out malignancy.
Borderline Ovarian Tumors: These tumors have a better prognosis than invasive ovarian cancers but can present similarly with cystic masses. The lack of solid components and low CA125 levels make this less likely, but it's a critical diagnosis not to miss.

Serous Cystadenoma: A type of benign ovarian tumor that can be septated. While less common, it's a consideration in the differential diagnosis of ovarian cysts in postmenopausal women.
Mucinous Cystadenoma: Another type of benign ovarian tumor, which can also present as septated cysts. These are less common than serous cystadenomas but should be considered in the differential diagnosis.