help with pain

i am at about 20 months post surgery. i finished out my pt with strength restored in my surgery leg at 99%. i can run squat lunge and what not but i recently started having pain in my kneecap. right at the bottom of the knee cap on my surgery leg. movement causes pain there but i can also recreate the pain by pressing down on that spot right at the bottom of my kneecap. does anyone know why my leg is doing this and what is causing it? most importantly is there any exercise or stretches that can make the pain go away?