Hi and thanks for the query,
Its important know you had ever had a problems with your knees or not. If its no, its therefore conclusive that the lesion at the knees is traumatic. Mild to moderate pain should be not a major concern, when left on its own. Accumulation of fluid at the level of the knees might deserve a more keener observation. The possibility of accumulation of blood or hemochromatosis
must be excluded. A past history of recurrent synovial fluid
collection must be considered
Traumatic hemochromatosis or fluid collection at the level of knees usually indicates rupture of cruciate ligaments that need to be treated urgently.
I suggest you consult your orthopedic surgeon in case it becomes really serious and on time too.
Thanks and best regards,