A 35-year-old woman presents with fever and night sweats for over 20 days. Her temperature is around 38°C. Physical examination reveals a distended abdomen with doughy consistency, mild tenderness and rebound tenderness diffusely. The liver and spleen are not palpable below the costal margin. Shifting dullness is positive. Laboratory tests show a white blood cell (WBC) count of 7.2×10⁹/L with 75% neutrophils (N) and 25% lymphocytes (L). What is the most likely diagnosis?