Swelling and knee pain

I am a 58-year-old woman in generally good health but I am 75 lbs. overweight. About 18 months ago I applied for a job which required some physical training. For most of one day I had to kneel (on grass) while practicing self-defense moves. My left knee, which I hurt in a skiing accident at age 15 (torn ligament), swelled up and I had trouble walking for nearly 2 weeks. Since that time I have had periodic flare-up of the same condition, often when I walk long distances (a mile or so when I take my dog for a walk). I am currently in Weight Watchers to lose weight, and hope that getting the excess weight off will help, but am concerned there might be a problem that needs tending to rather than waiting the year it will probably take to lose 75 lbs.

Thanks