Dysfunctional Uterine Bleeding (DUB): This is the most common cause of abnormal vaginal bleeding in women of reproductive age, especially in the absence of other symptoms. It is often related to hormonal changes, particularly anovulatory cycles, which are common in young women.

Pregnancy-Related Complications: Given the patient's age, pregnancy should always be considered. Complications such as miscarriage, ectopic pregnancy, or placenta previa could cause vaginal bleeding.
Infection: Pelvic inflammatory disease (PID) or other infections of the reproductive tract can cause bleeding.
Trauma: Sexual trauma or accidental injury to the genital area can result in vaginal bleeding.
Hormonal Contraceptive-Related Bleeding: Changes in or initiation of hormonal contraceptives can lead to breakthrough bleeding.

Ectopic Pregnancy: Although less common, ectopic pregnancy is a life-threatening condition that must be ruled out, especially if the patient is sexually active and has a positive pregnancy test.
Malignancy: Although rare in young women, cervical, uterine, or ovarian cancer can cause vaginal bleeding and must not be overlooked.
Blood Clotting Disorders: Conditions like von Willebrand disease can cause heavy or prolonged menstrual bleeding.

Adenomyosis: A condition where tissue similar to the lining inside the uterus is found outside its normal location, leading to heavy or painful periods, though it's less common in younger women.
Uterine Arteriovenous Malformation (AVM): A rare condition that can cause abnormal vaginal bleeding due to an abnormal connection between arteries and veins in the uterus.
Gonadal Dysgenesis: A condition related to the development of the ovaries, which can lead to hormonal imbalances and irregular bleeding, though it's typically identified earlier in life.
Thyroid Disorders: Both hyperthyroidism and hypothyroidism can affect menstrual regularity and cause abnormal bleeding, though they are less directly associated with vaginal bleeding compared to other symptoms.