Suggest Medication For Strained Knee

hi, i recently strained my knee while bending down and it seemed to be getting better after a week...but today it happened again very easily....hurt to bend back or forth a lot......i then straightened my leg as much as possbile..probably what you would call hyper extension or something...and then i felt a snap right under my knee and down a few inches.....it felt pretty good in comparison after...then it happend again this morning .....so to ease the pain ihyper striaghtened my leg until it snapped again ....pain eased up...i am worried as to what the cause of this is....i wasnt doing anything other then bending down which pretty much caused my knee to get hurt