I would strongly recommend that you see an orthopedic surgeon
to address this situation. Being young and active it is important to address this prior to permanent damage.
The knee is a very sensitive joint. While it has a lot of mobility, this leads to it requiring a synchrony of many difference parts. Bones, ligaments, muscles and tendons are all involved. A problem with any of these could be causing your symptoms.
Generally speaking, your symptoms of discomfort and swelling sounds likes inflammation. Inflammation can be caused by many different causes, however joint instability is the most common cause. Therefore, what likely happens that when you try and use the knee there is some unwanted movements that occur in the joint, thereby resulting in inflammation and thereby causing you symptoms. Therefore it is likely that there is some structural problem in your joint, like a ligament or meniscal tear. Another possibility is simply surrounding muscle weakness, another common problem in previously operated knees.
It is important to address this early. If a joint is allowed to continue with instability, eventually the bone becomes affected, which can cause arthritis
See an orthopedic surgeon at once. It sounds like you have a structural knee problem.