Ask a PT Response: "When you mention you are having pain at the back of the knees, do you literally mean that the pain is located at the posterior region (popliteal region of the knees) or are you having pain at the medial aspect (inside of knee) and lateral aspect (outside of knee). There are a number of factors which could contribute to knee pain and without physically examining you I would not be able to determine what could be going on. Nevertheless, It is odd that the pain develops sporadically with just walking. It may be a good idea for you to see a physical therapist so that he or she can conduct an evaluation to determine what may be going on at your knees. A postural assessment, gait analysis and other tests should allow your PT to gain a better understanding of your condition. Best of luck to you."