Hi and thanks for the query,
The most feared cause of a painful knee as such that is be excluded is a septic arthritis
An X ray
, full blood count and markers of inflammation should be enough to give a diagnosis. In this case, if its not a septic arthritis, other inflammatory causes like auto immune diseases, gout
could also cause such a pain.
A history of a neglected trauma also must be considered.
A visit to your family care physician for a proper evaluation and examinations shall be key in ,making an appropriate diagnosis.
thanks and best regards,