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.

LoseWtBodyGainWtLeg: With that kind of story best get that checked sooner than later.Yes, could be a blood clot,although unusual at your age.As for always being thirsty,that could be due to a lot of things & should be addressed by whomever you see for your leg.Congratulations on your weight loss but losing or gaining weight too fast puts quite a strain on your metabolism and can lead to problems.See you in the ER!!