A heavy pack can certainly cause problems. It has nothing to do with size but with weight. For ideas on lighening up, see the articles on the home page of this site, left hand column.

Strengthening the leg muscles is also important. Those muscles support your knee.

Finally, ask for a referral to a sports medicine specialist.
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey