Miscarriage: This is the most common cause of pelvic pain and bleeding in early pregnancy, especially at 8 weeks. The symptoms presented are classic for a miscarriage, which can be either threatened, inevitable, or complete.

Ectopic Pregnancy: Although less common than miscarriage, ectopic pregnancy is a significant concern due to its potential for severe morbidity and mortality. Pelvic pain and bleeding can be symptoms, especially if the ectopic pregnancy is ruptured.
Threatened Abortion: This condition presents with bleeding and sometimes pain but does not necessarily lead to miscarriage. It's a common cause of bleeding in early pregnancy.

Ruptured Ectopic Pregnancy: This is a medical emergency. While it might not be the most likely diagnosis, missing it could be fatal. Severe abdominal pain and significant bleeding are key symptoms.
Molar Pregnancy: Though rare, a molar pregnancy can cause significant bleeding and should not be missed due to its potential for complications, including gestational trophoblastic disease.
Placenta Previa: This condition, where the placenta covers the cervical os, can cause painless vaginal bleeding but can also be associated with pelvic pain if there is associated placental abruption.

Ovarian Torsion: While not directly related to pregnancy, ovarian torsion can cause severe pelvic pain and might be considered in the differential, especially if the pain is unilateral and severe.
Uterine Rupture: Extremely rare in early pregnancy unless there's a history of uterine surgery, this condition would present with severe pain and potentially life-threatening bleeding.