Lost A LOT of weight fast. Always very thirsty. Left leg is VERY swollen until halfway under knee. No marks. Is it a blood clot or something else?

2 conditions. The weight loss and thirstiness point towards diabetes, very common presentation and relatively easy to diagnose. The leg swelling is definitely of concern for deep vein thrombosis (DVT) or a clot in the venous circulation and a leg ultrasound should answer that question. Nephrotic syndrome (leaking protein in the urine) is a possible link and a predisposing condition for DVT.