Appendicitis: The sudden onset of severe right lower abdominal pain, tenderness, rebound tenderness, and guarding are classic signs of appendicitis. The patient's age and presentation are also consistent with this diagnosis.

Ovarian Torsion: The severe pain and sudden onset could be indicative of ovarian torsion, especially given the patient's gender. This condition requires prompt surgical intervention.
Ectopic Pregnancy: Although less likely if the patient is not known to be pregnant, ectopic pregnancy can present with severe abdominal pain and should be considered, especially if there's a possibility of pregnancy.
Ruptured Ovarian Cyst: A ruptured ovarian cyst can cause severe pain and may lead to hemodynamic instability, fitting the patient's presentation.

Ruptured Ectopic Pregnancy: This is a life-threatening condition that requires immediate diagnosis and treatment. Even if the patient denies pregnancy, it should be considered due to the severity of symptoms.
Intestinal Obstruction: Although less common in young adults without prior abdominal surgery, intestinal obstruction can cause severe abdominal pain, vomiting, and signs of peritonitis.
Mesenteric Ischemia: This condition can present with severe abdominal pain out of proportion to physical findings and can lead to bowel infarction if not promptly treated.

Endometriosis: While endometriosis can cause chronic pelvic pain, it is less likely to cause acute, severe pain leading to syncope.
Inflammatory Bowel Disease (IBD) Flare: Although IBD can cause severe abdominal pain, it typically has a more chronic course and is less likely to present with such an acute onset.
Primary Peritonitis: This condition is rare in immunocompetent individuals but can occur, especially in females, and presents with diffuse or localized abdominal pain and signs of peritonitis.