It seems you have taken walking as a weight reduction method along with having food supplements. You might have strained the cartilages inside the knee. Before undergoing strenuous exercise programme it's necessary to tone up your basic postural muscles. Consult with a Physiotherapist who can give you an aerobic exercise programme in non weight bearing positions coupled with upper limb activities. Also seek professional help from a dietitian for a balanced diet based on your metabolic need.