It is not uncommon to experiences some aches and pains, particularly at the front of the knee during this stage of rehab.
As patients start to resume their normal, everyday activities, it is easy to "overdo it." The quadriceps "endurance strength" is still recovering and building up, but when we engage in activities beyond its capacity, we tend to overload our knee cap... hence, the achey pain.
It is okay. Recovery is a bit 5 steps forward, 1 step backward. Just listen to your symptoms and tone it done a bit when you realize you have overdone it. Usually it is the simple things, like standing too much or being on your feet too much throughout a given day that can sneak up on us.