Swelling of one leg from knee to foot? Can it be a clot?

Yes-go to ER ASAP. Your description is very suspicious for a blood clot called a deep venous thrombosis (dvt). You should go to the er asap for evaluation. The clots can move or form in your lung(s) becoming what is called a pulmonary embolism, which can be fatal (can kill you). Get it checked out now--this may be a potentially life threatening emergency.
It depends. Most blood clots that we see are in the posterior calf region. In the front of the lower leg, if there is swelling can be other issues such as shin splints or tendinitis. If you have been sedentary, smoke, take birth control, have been doing prolonged sitting, or having shortness of breath, you should have this evaluated with your family physician.