Perimenopausal hormonal changes: This is the most likely diagnosis due to the patient's age, as irregular bleeding is a common symptom during the perimenopausal transition due to fluctuating hormone levels.

Uterine fibroids: These are common in women of this age group and can cause irregular bleeding due to their location, size, and effect on the uterine lining.
Endometrial polyps: Similar to fibroids, polyps can cause irregular bleeding by disrupting the normal endometrial lining and its shedding process.
Hormonal imbalance: Conditions such as polycystic ovary syndrome (PCOS), thyroid disorders, or other endocrine imbalances can lead to irregular menstrual bleeding.
Intrauterine device (IUD) complications: If the patient has an IUD, especially a non-hormonal one, it could be a cause of irregular bleeding.

Endometrial cancer: Although less common, endometrial cancer is a critical diagnosis not to miss, as it can present with irregular vaginal bleeding in perimenopausal women.
Cervical cancer: Similarly, cervical cancer can cause irregular vaginal bleeding and must be ruled out with appropriate screening.
Pregnancy complications: Although less likely at 45, ectopic pregnancy or a molar pregnancy can cause irregular bleeding and are medical emergencies.

Bleeding disorders: Conditions like von Willebrand disease can cause heavy or irregular menstrual bleeding but are less common.
Adenomyosis: A condition where tissue similar to the lining inside the uterus grows into the muscular walls of the uterus, leading to irregular bleeding and pelvic pain.
Uterine sarcoma: A rare type of cancer that can cause irregular vaginal bleeding among other symptoms.