If the runny nose started after fall - it could be due to CSF rhinorrhoea
It is the condition when the Cerebro Spinal Fluid or CSF (the fluid in brain and spinal cord
) leaks through a gap / fracture
(which may be caused due to traumatic head injury
) in skull.
Most often, the runny nose is one sided ( watery fluid coming out of one nostril only).
Clinical examinations and tests (like Beta-2-Trasferrin test, glucose test) are there to detect if the fluid is CSF.
In most cases, CSF-rhinorrhoea stops spontaneously. If not stopped by 7-10 days, may need corrective surgical management.
Get your child examined by doctor and take treatments as necessary. Other injuries / concussion
, if present - will be detected.
Hope I could make it clear. Post if you have other queries.