What Causes Tiredness And Aching Of The Feet And The Knees?

Hi I am a 28 year old female, I work on the auto industry as an administrative and visit the plant several times a day. I practice yoga for 40 mins daily, eat healthy 4 times a day, and don´t have weight issues. Every morning I feel very tired, my feet are aching and tired, and as the day goes by I feel better, up until the evening when I start feeling very tired again. When I pass more than an hour sitting my knees hurt too, why could this be??