Dear Sir/Madam
I have gone through your query and read your symptoms.
In my opinion, normal or low weight can not cause knee problem due to weight, and that is usually
osteoarthritis, your age is 20 and this possibility is highly unlikely, if your knee have hyper mobility of
deformity, than even normal transmission of weight can cause it to
fatigue and painful.
you will need,
1 X rays to see the alignment and status of your knee joints.
2 Blood investigations to see any cause of generalized weakness.
3 if both are within normal limit,
physiotherapy to strengthen the weak muscles, and guided use can help you a lot.
I hope that answers your query. If you want any more clarification, contact me back.
Thanks
Dr Narender Saini