Does it feel like a muscle ache or a bone ache? Is it accompanied by any feeling of numbness? If it is a muscle ache with any numbness, it could be sciatica from being too inactive. If it is sciatica, it can usually be reversed with physical therapy (a prescribed set of exercises to rebalance muscle tone so that the muscles do not pinch a nerve). I recommend that you see a doctor to rule out other causes.
What could this terrible aching pain from my left hip all the way down to my knee be? It is worse when I am inactive.
Where am I?
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If it is along the outside of the hip to the hip down to the knee it could be a tight IT Band. It could be quads. Stretch your quads, IT Band and use a foam roller for your IT Band. You can Google these easier than I can describe them. You're best bet is to follow up with an evaluation by a physical therapist. In most states you can call a therapist for evaluation without having to see a doctor first.
See your doctor to rule out a possible DVT or other vein problems. When you are moving about, the leg muscles help the blood flow by constricting the vessels and prevent backflow of blood. Inactivity may be allowing the blood to pool and expand the vessels.