Suggest Remedy For Pain In Left Knee

Hi Doctor, I am 23yrs old,male, employee at a Software Industry. Since 3 months, I am having serious pain in my left knee, to an extent that I am unable to stand and walk for sometime, when I sit for 15mins with my knee bend. All these started, when after a gap of 6 months, I resumed to play badminton. I know that the pains would be there in such cases, but to such an extent is certainly alarming considering my age. The pain reduced after a month, but again recently when I was doing physical exercise like few jumpings, I again have that pain since last 10 days. Same left knee with the same pain. I am a pure vegetarian, no smoking and drinking habits!! Height:5 10. Weight: 78Kg.. Please advice as I want to do some physical exercise to reduce my weight and also be fit. Thanks!!