Possibly the weakness and pain in your knees is due to a weak quadriceps, I suggest that you start a consistent exercise routine aimed to strengthen the leg muscles, you can ride a bike 2 or 3 times a week and also do series of squat exercises at home. Walking for 30 to 45 minutes at least three times a week may be useful too in your case. Regarding your low weight, I suggest a consultation with a nutritionist, who will give you a healthy diet so you can gain some weight.
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