Knee pain, swelling, and giving way two years after ACL repair are signs and symptoms that the joint is unstable. If you haven't gone back to your orthopedic surgeon, now would be a good time to make an appointment.
It may be a simple case of muscular weakness or imbalance. Sometimes such problems can be taken care of with a rehab program. In other cases there may be other (unknown) damage to the joint. Perhaps there's a torn meniscus or some osteoarthritis developing.
Worst-case scenario: the repaired ACL may have failed. Further testing is needed to find out what's wrong. The chances are good that treatment is available that can get you back to the activities you like.