It can fit into those categories, but it can also fit into the calisthenics (e.g. pushups, situps, pullups, jumping jacks), heavy, vigorous effort, mild stretching,  stretching, hatha yoga, teaching aerobic exercise class,  weight lifting (free weight, nautilus or universal)  vigorous effort categpries as ...

Alana, Without actually knowing the exercise that you had difficulty with, I'll make the assumption that it was possibly the single leg squat, or single leg balance lunge.  While 20 extra lbs can definitely contribute to knee pain, more than likely it was your strength level and form that did you in.  There is also the ...