This happened to me my left thigh became numb for no apparent reason for 2 days and I went and saw a doctor he said it should go away but could take up to a month. after a week it returned to normal.
Are you sitting alot? If your leg is not sitting on the floor, but hangs, even slightly, you could be causing a partial block to a blood vessel in the back of the knee. When you get up your leg feels "heavy" and "sluggish" or slightly swollen without looking swollen. You can "walk it out" after a couple of minutes. If this is the problem, adjust your sitting position or chair so that your legs to promote circulation of blood in the legs. If this doesn't work, you may wish to see your health care provider and get checked out for varicose veins or get an xray to rule out congestive heart failure.