Hello dear, The knot formation in mid/lower fibula after injury is due to accumulation of fluid & blood and it is called Hematoma. The blood cells accumulated are eventually broken down and absorbed in the body. But this is a slow process & takes times to disappear completely. Till that time, symptomatic relief is obtained by analgesics preparations, ice compression & adequate rest. If it still persists or there are any other associated symptoms like inflammatory changes, loss of sensation, tingling, etc...then it will be better to consult her Pediatrician/ Orthopedician. Under such circumstances, a proper clinical+ radiological evaluation will be needed to rule out any pathological cause for symptoms. Till then, maintain adequate hydration & nutrition status. There is no need to worry, she will be fine. Wishing your daughter a good health. Take care.