Appendicitis: This is often the first consideration for fever accompanied by abdominal pain, especially if the pain is localized to the lower right quadrant. The combination of fever, abdominal tenderness, and guarding (muscle tension) is highly suggestive of appendicitis.

Gastroenteritis: Inflammation of the stomach and intestines, usually due to infection, can cause fever, abdominal pain, diarrhea, and vomiting. It's a common condition that can be viral, bacterial, or parasitic in origin.
Urinary Tract Infection (UTI): While UTIs are more commonly associated with dysuria and frequency, they can also present with abdominal pain and fever, especially if the infection has ascended to the kidneys (pyelonephritis).
Diverticulitis: Inflammation of the diverticula in the colon can cause fever, abdominal pain (usually lower left quadrant), and changes in bowel habits. It's more common in older adults.

Ectopic Pregnancy: Although less common, an ectopic pregnancy can present with abdominal pain and fever, especially if there is associated bleeding or rupture. This is a medical emergency.
Perforated Viscus: A hole in the wall of the stomach or intestine can lead to peritonitis, an infection of the abdominal cavity, which presents with severe abdominal pain, fever, and possibly sepsis.
Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain, fever, and vomiting. It can be life-threatening if not promptly treated.
Intestinal Obstruction: Blockage of the intestine can lead to severe abdominal pain, fever, vomiting, and constipation. It requires immediate medical attention.

Mesenteric Lymphadenitis: Inflammation of the lymph nodes in the mesentery (the tissue that attaches the intestines to the back of the abdominal wall) can mimic appendicitis but is less common.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease and ulcerative colitis can cause fever and abdominal pain during a flare-up, though they are typically associated with chronic symptoms like diarrhea and weight loss.
Abdominal Tuberculosis: A rare form of tuberculosis that affects the abdominal organs, which can cause fever, abdominal pain, and weight loss.
Ovarian Torsion: Twisting of the ovary can cause severe abdominal pain and fever, though it's more commonly associated with pelvic pain and is a surgical emergency.