Spontaneous Abortion (Miscarriage): This is the most likely diagnosis given the symptoms of severe pain in the lower abdomen, heavy vaginal bleeding, and an empty uterus at 11 weeks gestation. The cervix being dilated further supports this diagnosis, as it indicates the body is in the process of expelling the pregnancy.

Ectopic Pregnancy: Although less likely given the empty uterus, an ectopic pregnancy that has ruptured could present with severe abdominal pain and heavy vaginal bleeding. However, the presence of an empty uterus makes this less likely.
Threatened Abortion: This condition involves vaginal bleeding with or without abdominal pain, but the cervix is closed, and the fetus is still viable. Given the cervix is dilated and the uterus is empty, this diagnosis is less likely but could be considered if there were any doubts about the completeness of the miscarriage.

Ruptured Ectopic Pregnancy: Although mentioned earlier, it's crucial to emphasize that a ruptured ectopic pregnancy is a medical emergency. Even though the uterus is empty, a very small chance exists that the ectopic pregnancy ruptured and was not detected, or there could be a heterotopic pregnancy (a simultaneous intrauterine and ectopic pregnancy), which is rare but more common with assisted reproductive technology.
Uterine Rupture: Though extremely rare at this gestational age, uterine rupture could present with severe abdominal pain and vaginal bleeding. This would be more concerning in a patient with a previous uterine scar.

Molar Pregnancy with Sepsis: A molar pregnancy can cause severe bleeding and abdominal pain, but it's less common and typically presents earlier in pregnancy. Sepsis could complicate any of the above conditions if there is retained products of conception or infection.
Heterotopic Pregnancy: As mentioned, this is a rare condition where there is both an intrauterine and an ectopic pregnancy. Given the empty uterus, this would be extremely unlikely unless the intrauterine pregnancy was also failing or had been missed.