Ok, thank you for the additional information.
There are several things that can cause swelling of one leg..
The most serious of these is a blood clot.
While you do not appear to have any risk factors for the development of a blood clot, that does not mean that you cannot develop one.
As you may know, a blood clot in the leg is a serious condition given the fact that it could break off and travel to the lung
While this may not be what is going on, there is no way to know whether this is the cause without an exam and an ultrasound of the veins of the affected leg.
I do recommend that you have this checked out today in the emergency room given the fact that, if it were to be a blood clot, it is a potentially serious/life threatening condition. But a treatable one.
Other causes of swelling of the leg include varicose veins, muscle pulls/tears, bakers cysts (which occur behind the knee and can rupture), skin infections of the leg, poor lymph flow of the leg and medication induced swelling (statins do not cause this).
Please feel free to ask any follow up questions you may have after reading this,