Get to a doctor NOW.
I had the same thing two years ago. It turned out to be a DVT or deep vein thrombosis. These clots can break off and lodge in your heart lungs or brain. This can be fatal.They do a doppler test to find it. Better that it is in the lower leg than in the thigh.
My clot is gone now, but, I will be on blood thinners for the rest of my life. I go to the Coumadin Clinic every few weeks. They do a finger-stick and based on how thick or thin my blood is, then, they change my Coumadin dose or not.
If it is not a clot, better safe than sorry.