Replying to The back of my knee

first off i did searchbar and couldnt find anything similar to mine.

so a week ago i landed sideways and tweaked my knee.
it hurt pretty good at first but i could ski it off slowly.
my knee still hurts but only when i straighten my leg as a walk.
it hurts on the back of the knee
its not a really bad pain which is why i havent seen a doctor or anything yet, but its not how my knee is supossed to feel.
im probably going to see a doctor soon but anyone have a clue what could be the problem?