Intussusception: This condition is characterized by the telescoping of one portion of the intestine into another, leading to bowel obstruction and potentially ischemia. The presentation of bilious vomiting, colic, and a small tinge of blood in the stool (currant jelly stool) is classic for intussusception, especially in an infant of this age group. The recent history of a UTI treated with antibiotics could be coincidental, but it's also possible that the infection or the antibiotic treatment might have contributed to the development of intussusception, possibly through an alteration in gut flora or an immune response.

Gastroenteritis: Although the infant is afebrile, a viral or bacterial gastroenteritis could explain the symptoms, especially if the episode of bilious vomiting was an isolated event. The presence of blood in the urine could be due to a concurrent urinary tract infection or another cause of hematuria.
Urinary Tract Infection (UTI): Given the recent history of a UTI, it's possible that the infection was not fully cleared or has recurred, especially with the presence of blood in the urine. However, the absence of fever and the specific gastrointestinal symptoms suggest that this might not be the primary issue at the moment.
Infectious Gastroenteritis with Urinary Contamination: The blood in the diaper could be from gastrointestinal bleeding contaminating the urine, rather than a primary urinary issue.

Volvolus: A condition where the intestine twists around its mesentery, leading to obstruction and potential ischemia. It presents similarly to intussusception with bilious vomiting and abdominal pain but requires immediate surgical intervention.
Hirschsprung's Disease: A congenital condition characterized by the absence of ganglion cells in the distal bowel, leading to obstruction. While it typically presents in the neonatal period, some cases may not become apparent until later. The presence of bilious vomiting and abdominal distress could be indicative of this condition, especially if intussusception is ruled out.
Septicemia or Bacteremia: Although the infant is afebrile, the recent history of a UTI with a potentially virulent organism (Citrobacter koseri) and the current symptoms could suggest a systemic infection, which would be critical to identify and treat promptly.

Meckel's Diverticulum: A congenital anomaly of the small intestine that can cause gastrointestinal bleeding, obstruction, or diverticulitis. It's a rare cause of these symptoms in infants but should be considered if other diagnoses are ruled out.
Congenital Anomalies of the Urinary Tract: Certain anomalies could lead to recurrent infections, obstruction, or other complications that might explain the infant's symptoms, including the hematuria.