hello,,thanks for query
tenderness,swelling on the knrr after an injury
cannot be neglected..all the injuries of the kneeand hair line fractores may not be picked up by x ray(x ray detects bony injuries)..
injury can be bony,muscular,ligamentous,tendon injuries or collection inside a joint
so suggest you to consult an orthopaedician immediately and follow his treatment
get well soon