Appendicitis: This condition is a common cause of lower abdominal pain accompanied by fever. The pain often starts near the navel and then moves to the lower right abdomen. The fever, in this case, 38.1°C, supports the presence of an infectious or inflammatory process, which is consistent with appendicitis.

Urinary Tract Infection (UTI): While UTIs can present with a wide range of symptoms, lower abdominal pain and fever are common, especially in cases of pyelonephritis (infection of the kidney). The presence of fever suggests an upper UTI, which could involve the kidneys.
Diverticulitis: This condition involves inflammation of the diverticula in the digestive tract, often presenting with lower abdominal pain (usually on the left side) and fever. It's more common in older adults but should be considered in the differential diagnosis.
Ovarian Cyst or Torsion: In females, ovarian cysts can cause lower abdominal pain, and if the cyst ruptures or undergoes torsion, it can lead to severe pain and fever due to potential infection or necrosis.

Ectopic Pregnancy: Although less common, an ectopic pregnancy is a life-threatening condition that can present with lower abdominal pain and fever. It's crucial to consider this diagnosis in females of childbearing age, especially if there's a possibility of pregnancy.
Perforated Viscus: A perforation in the gastrointestinal tract can lead to peritonitis, an infection of the abdominal cavity, which presents with severe abdominal pain and fever. This is a surgical emergency.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease or ulcerative colitis can cause lower abdominal pain and fever during a flare-up. While not immediately life-threatening like some other conditions, managing IBD is crucial to prevent complications.

Endometriosis: While endometriosis can cause chronic pelvic pain, it's less commonly associated with acute fever. However, in rare cases, endometriotic cysts can become infected, leading to acute pain and fever.
Tuberculosis: Abdominal tuberculosis can cause lower abdominal pain and fever, among other symptoms. It's more common in areas where TB is prevalent and in immunocompromised individuals.
Yersiniosis: A bacterial infection caused by Yersinia enterocolitica, which can mimic appendicitis with lower right quadrant pain and fever. It's less common but should be considered, especially in areas with outbreaks or in immunocompromised patients.