from your description it appears that you may have anterior knee pain which sometimes presents as pain at the back of the knee. the second possibility is that you have over done your exercise and hamstrings are sore and the lower end of the knee cap is inflamed. either way you should rest and ice the area of pain and give it time. i suggest you add nsaids (nonsteroidal anti-inflammatory drugs) for a week for an additional anti-inflammatory action. for further information consult an orthopaedician and traumatologist online --> <link>
is it fine to exercise with knee pain?