Anovulation: This is the most likely diagnosis given the patient's irregular vaginal bleeding pattern, which includes frequent bleeding and an episode of amenorrhea. Anovulation leads to irregular endometrial growth and shedding, resulting in unpredictable bleeding patterns. The presence of proliferative endometrium on biopsy supports this diagnosis, as it indicates that the endometrium is being stimulated by estrogen but not undergoing the usual progesterone-induced secretory changes due to the lack of ovulation.

Uterine Fibroid: Although the pelvic ultrasound findings are normal, small fibroids might not be detected, and they can cause irregular vaginal bleeding by disrupting the normal architecture of the uterus and affecting endometrial growth and shedding.
Cervical Polyp: While less likely given the normal pelvic examination findings, a cervical polyp could potentially cause irregular bleeding, especially if it is large enough to interfere with normal cervical function or if it undergoes necrosis and bleeding.

Endometrial Cancer: Although the endometrial biopsy shows proliferative endometrium, which is not diagnostic of cancer, endometrial cancer must be considered in the differential diagnosis of irregular vaginal bleeding in any woman, especially given the patient's age and symptoms. A single negative biopsy does not entirely rule out the possibility of cancer, especially if the sampling was inadequate or the cancer is focal.
Menopause: While the patient's age and symptoms could suggest perimenopause, the presence of frequent bleeding and a negative pregnancy test, along with the biopsy findings, make this less likely. However, menopause or perimenopause should be considered, as it can cause irregular bleeding patterns.

Other rare causes of irregular vaginal bleeding, such as thyroid disorders, pituitary tumors, or coagulopathies, could be considered but are less likely given the information provided. These conditions can affect menstrual regularity and should be investigated if initial evaluations for more common causes are negative and the patient's condition does not improve with treatment for anovulation or other identified causes.