the sharp pain was probably a minor sprain. Your ligament probably just get stressed in that motion. This is once again usually caused by weak muscles. Your muscles are your first line of defense, if they fail to protect the joint, the ligaments are next. they are backup. I would suggest doing a lot of muscle work around the knee joint, especially massaging on the back of your knee. To do that, bend your knee while sitting, and get your thumbs right in what is called the popliteal fossae, and rub the heck out of that are, then try it.
a minor sprain? phew thank god, i thought it might be arthritis.
so the reason why i feel stiff is because my muscles failed to protect my knee joint, resulting in my ligament protecting the joint, and the ligament got stressed?
and, erm, what exactly is the popliteal fossae
i google imaged it and no helpful diagrams there
and by strengthening the muscle around the knee joint, do you mean the hamstrings aswell?
one more question: is rubbing this popliteal thingy the actual exercise or just to massage it, and if it is just to massage it what can I do to exercise the muscles there?
sorry for all the questions, i just really want to be sure what you mean, and what i'm doing.
thanks for all the insight.