What Causes Burning Sensation In The Left Ankle?

I am 48 years old, 5 ft 9 and 175 lb... This evening I got up out of a chair and walked to my car and drove 10 miles. I went to get out of my car and walk... I felt a burning, throbbing pain above the inside of my left ankle. When I looked at it, I seen there is a lump the size of a golf ball. No marks, no bites no redness... just this lump. It started out a burning, throbbing pain and has now progressed into numbness and tingling along with the burning and throbbing. It hurts to put weight on it and when I raise it when sitting down. When I stretch my leg out, there is a tightness and a streak of pain which causes the throbbing to be extreme. But if I sit in a chair with my leg down with no weight on it, the pain is a lot less, but it still throbs. Could this be a clot ? Should I go to the emergency room? Or should I give it time, and take something for the pain? This has never happened before and I am not sure what to do.