I hope I can help you here, and I am so very sorry you are in such discomfort. Please understand that on line no one can state anything with certainty or diagnose; I can only give my best guess based upon what you wrote.
It is quite possible that you are describing something called "Intermittent claudication" Intermittent claudication refers to pain that is more typically described as burning, rather than aching and its hallmark is that it comes on during some sort of activity...such as walking. Also, the leg can be cooler and sometimes pale, which you do not describe but it does not mean you do not have this. Intermittent claudication is a result of peripheral vascular disease (PVD). In this, veins get partially or totally blocked, (yes, could be a clot) preventing oxygenated blood from feeding the tissues. Pain is the result of tissues that are starved for oxygen. If this PVD, it can range from extremely mild to severe cases that cause ulcers and even gangrene.
If this is the case, a doctors evaluation is called for; and I would call your provider tomorrow to schedule one. Untreated PVD can be potentially quite risky. I suggest that you make an appointment to be seen by your doctor regarding this...he may want to run some tests like a ultrasound or CT or MRI to make sure this discomfort is not caused by PVD.
IF the pain were to become severe, agony, excruciating today, that would be a very good reason to be seen in the Emergency Room.
Here is further information regarding this: http://www.vascular.co.nz/intermittent_claudication.htm