Miscarriage: This is the most likely diagnosis given the context of the question. Lower abdominal pain is a common symptom associated with miscarriage, which is the loss of a fetus before the 20th week of pregnancy. The pain can range from mild to severe and is often accompanied by vaginal bleeding.

Ectopic Pregnancy: Although less common than miscarriage, ectopic pregnancy is a significant consideration in any woman of childbearing age presenting with lower abdominal pain and a history of miscarriage or irregular menstrual cycles. It occurs when a fertilized egg implants outside the uterus, often causing severe pain and potentially life-threatening complications.
Pelvic Inflammatory Disease (PID): PID can cause lower abdominal pain and is a possible diagnosis, especially if there is a history of sexually transmitted infections or previous pelvic infections. However, it might not be directly related to miscarriage unless the miscarriage was complicated by infection.
Ovarian Cyst Rupture or Torsion: These conditions can cause acute lower abdominal pain and might be considered, especially if the pain is sudden and severe. They are not directly related to miscarriage but could be a coincidental finding.

Ruptured Ectopic Pregnancy: This is a medical emergency that requires immediate attention. The symptoms can be similar to those of a miscarriage, but the severity of the pain and the presence of hemodynamic instability can indicate a ruptured ectopic pregnancy.
Appendicitis: Although not directly related to miscarriage, appendicitis can cause lower abdominal pain and must be considered, especially if the pain is localized to the right lower quadrant. Missing this diagnosis can lead to severe complications, including perforation and peritonitis.
Placental Abruption: In a pregnant woman, placental abruption (where the placenta separates from the uterus) can cause severe abdominal pain and vaginal bleeding. It's a life-threatening condition for both the mother and the fetus.

Molar Pregnancy: A rare condition where a non-viable fertilized egg implants in the uterus, leading to complications. It can cause abdominal pain and vaginal bleeding but is much less common than miscarriage.
Degenerating Uterine Leiomyoma (Fibroid): Fibroids can cause abdominal pain, and if they degenerate, the pain can become acute. However, this is less likely to be directly related to miscarriage unless the fibroid is large and pedunculated, causing torsion or degeneration.