Visible blood in the urine or stool should always be assessed by a doctor, even if it happens only once and there is no pain, because it can be an early sign of conditions that are easier to treat when caught early. Causes range from infections, hemorrhoids, and kidney stones to inflammatory and more serious conditions, so a proper evaluation is important.
Seek urgent or emergency care if the bleeding is heavy, if stools are black and tarry or look like they contain large amounts of blood (which can mean bleeding higher in the gut), or if bleeding comes with severe abdominal or back pain, fever, dizziness, paleness, a racing heart, or fainting. These suggest significant blood loss or a serious underlying cause and need prompt treatment.
Even small amounts of blood, or blood you only notice occasionally, deserve a non-urgent medical appointment rather than being ignored. Mention any blood-thinning medication you take, as it can affect both the cause and the urgency.