Weight bearing knee pain on one knee, i walk about eight miles a day at work and am very active. no swelling or tightness. ?

Many possibilities: Hi, weight bearing knee pain can be many things. If the pain is anterior & especially if there is pain descending stairs then patellofemoral joint is the likely culprit & malalignment is not uncommon in younger people, esp. women. Other considerations: cartilage, meniscus, tendinitis, stress fx, and ligament injury. Ligament injury is not likely since you do not advise of trauma-imaging & MD eval