Hmmm...maybe plantar fasciitis? Here's a description of the symptoms: Most people with plantar fasciitis have pain when they take their first steps after they get out of bed or sit for a long time. You may have less stiffness and pain after you take a few steps. But your foot may hurt more as the day goes on. It may hurt the most when you climb stairs or after you stand for a long time.
It could also be something as simple as tight calf muscles. Some simple stretches can help (you'll want to stretch with the knee bent a bit and straight as well. Massage can help, as can warm bath with Epsom salts.