What causes knee pain when bending my knee?

i was getting my 9 month old out of the car when i got sudden sharp pain in my knee...the pain didnt occur with walking, mostly just with getting up and down from sitting and going up and down stairs. i also had pain when scooting my chair forward or backward while sitting in it. basically, i have pain when bending my knee and putting weight on it...the pain subsided by the time i went to see my doctor (general practitioner). about a week after the initial pain, i began a long drive (17 hr trip) and the pain started to return...i also started to have swelling and pain in general just with movement..i would wake up with pain every time i moved during the night...by the 2nd day of the trip, i could not bend my knee enough to put my sock on...now, 5 days later, the pain is subsiding again...anyone know what could be the problem?