What Causes Pressure Under Knee Caps While Walking?

I have this weird pressure under my knee caps and i get it about 4 times a week.. its mainly when i walk and the pressure dissipates when i sit. but it feels as if my knee caps are going to burst off of my legs.. what could this be? and yes i have patellofemoral syndrome and flat feet but it isnt the same pain and the pressure isnt painful just uncomfortable