Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Get emergency help for severe trouble breathing, chest pain, fainting, confusion, extreme weakness, or rapidly worsening paleness.
Seek prompt medical advice if an infection is followed by a clear increase in weakness, jaundice, breathlessness, or paleness.
These symptoms may mean the anemia has worsened.
A newborn with yellow skin or eyes should receive same-day medical assessment.
Seek urgent help if the baby is difficult to wake, feeds poorly, has an unusual cry, or appears increasingly yellow.
These can be signs of severe anemia or another urgent condition.
Follow your individual fever plan and seek urgent medical assessment immediately rather than waiting for a routine appointment.
And for anyone, someone who:
Arrange a routine appointment if you develop new or gradually worsening fatigue, paleness, yellowing of the eyes, dark urine, or discomfort under the left ribs.
Repeated upper abdominal pain, especially after meals, may need assessment for gallstones.
The article also says
If you have had your spleen removed, fever can signal a serious infection.
Every sign above is quoted from the sofidex article on Hereditary elliptocytosis. If you are not sure, err upward — it is always reasonable to be checked.