What Causes Knee Pain While Doing Exercise?

I tore my ACL in Jan 2010. No surgery brace for 3 months and smaller brace as needed. My recovery is great. I have been working out all winter. walking and fitness video's mostly low impact. I have also been doing some strenght training vidoes lower body which means lunges and knee lifts. I have noticed some changes in my knee stability. I am wanting to know if I can damage my pcl without a strong blow to the front of my knee. My age is 55. I have always been very active however no more skiing.