Appendicitis: This is a common cause of acute abdominal pain, especially in young adults. The pain often starts near the navel and then moves to the lower right abdomen. Given the sudden onset of severe abdominal pain, appendicitis is a plausible diagnosis.

Ovarian Cyst Rupture or Ovarian Torsion: These conditions are common in women of reproductive age and can cause severe, sudden abdominal pain. The pain is often localized to one side of the lower abdomen.
Gastroenteritis: Although it typically causes less severe pain, a severe case of gastroenteritis could present with significant abdominal pain, especially if there's an inflammatory or infectious component.
Urinary Tract Infection (UTI) or Pyelonephritis: While UTIs are more common, pyelonephritis (an infection of the kidney) can cause severe flank pain that may be perceived as abdominal pain.
Diverticulitis: Although less common in younger adults, diverticulitis can cause severe abdominal pain, usually on the lower left side.

Ectopic Pregnancy: This is a life-threatening condition that must be considered in any female of childbearing age presenting with abdominal pain, even if she reports not being pregnant or using contraception.
Ruptured Aortic Aneurysm: Although rare in young adults, a ruptured abdominal aortic aneurysm is a medical emergency that requires immediate attention.
Intestinal Obstruction: This condition can cause severe abdominal pain and is a surgical emergency if not promptly addressed.
Mesenteric Ischemia: This condition involves insufficient blood flow to the intestines and can cause severe abdominal pain out of proportion to the physical examination findings.

Endometriosis: While endometriosis can cause chronic pelvic pain, it rarely causes acute, severe abdominal pain unless there's a complication like an endometrioma rupture.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease or ulcerative colitis can cause severe abdominal pain during a flare, but the onset is often more gradual and accompanied by other symptoms like diarrhea or weight loss.
Pancreatitis: Although more common in older adults or those with a history of gallstones or alcohol use, pancreatitis can occur in younger adults and causes severe, persistent abdominal pain.
Hepatic or Splenic Infarction: These conditions are rare and usually occur in the context of specific risk factors (e.g., sickle cell disease, antiphospholipid syndrome), but they can cause severe abdominal pain.