What Causes Swelling And Painful Bump On Knee After An Injury?

i am 15 years old i play volleyball very competitively, i have had 2 bilateral fractures in my lower back and 3 completely torn ligaments in my ankle i have done a lot of rehab and just got back to playing last week. i consulted my doctor when i was first casted because of my ankle they said i had just sprained my knee when i went down, the pain was around the inside a little below my knee cap. as i started back practicing the outside of my knee all along the side swelled and is still swollen. when i press on it it is very tender and there are some spots that send shocks almost and feel like it is a little bump floating around there are multiple spots where i feel this. also if i am sitting for a long time and start to walk again i feel again like something is moving in the outside of my knee. any help at all would be great! thanks!