There is a pain between my kneecap and where the shin ends and it happens while i sit down. It's just under (inside) the kneecap. I was wondering what could it be.

It's not always there. Usually there is no pain even after my exercises. But today i was doing body weight squats when it happened. Since i knew my knee was weak, i only did 10 repeat ( ) and then i started to stretch (the one with one leg in front of you, like lunging) and there was the pain.

PS : Sorry about the weak explanation. That's the best i can do in English..

It could be a number of things. The only way to know for sure is to seek medical attention. If you had a warning light flashing on the dashboard of your car you would want to get your car to a mechanic. Take care of yourself.

If you are doing foward lunge stretches, your knee should not go past your ankle. Your ankle should be right under your knee. If you have pain, usually you should be resting/recovering and not trying for 10 more reps. Try wearing a compression knee brace. The heat will help your knee. Also, you could take glucosamine supplements. They will strengthen your joints.