You might have Patellar tendinitis:
Do you mean the area below the kneecap?
If you mean literally inside/within the kneecap, it will probably be labelled as "Chrondomalacia Patella" which is a somewhat non-descript term, suggesting a painful problem at the kneecap.
I had it to an extreme amount, it seemed to be due to unbalanced stress from various muscles. It was fixed by getting squat shoes. Some people (like Gary Gibson) felt the opposite, that shoes gives their knees more trouble than flat feet.