What would make my legs ache from my knees down?

It could be a number of things, meaning you must seek professional medical advice, time to see a Dr..

"Leg pain is a symptom with many possible causes. Most leg pain results from wear and tear or overuse. It also can result from injuries or health conditions in joints, bones, muscles, ligaments, tendons, nerves or other soft tissues. Some types of leg pain can be traced to problems in your lower spine."

Source: Mayo Clinic