DVT stands for deep vein thrombosis: Blood clotting in the veins of the thigh or leg. In air travel, DVT is the "economy-class syndrome." Even in young, healthy travelers the long stretches immobilized in cramped seats in cabins with very low humidity set the stage for the formation of a thrombus (blood clot) in the lower leg. Blood clots can break off (as emboli) and make their way to the lungs where they have the potential of causing respiratory distress and respiratory failure. Those who have hip or knee replacement surgery are also at higher risk in the short term and usually blood-thinning drugs are given or other measures taken to lower that risk during the immediate post-op period. One of the possible symptoms of DVT is swelling. Other symptoms, depending on the size and location of a DVT, can include:
warmth, redness, and /or pain in a leg.
DVT of the calf may cause symptoms in the calf while that in the thigh can cause symptoms in both the thigh and/or calf.
The vast majority of DVTs occurs in only one leg at a time.


These are but two of the conditions that could cause swelling. There are many others. Take a look here for a more complete discussion of edema and conditions that could cause it. Please note that there are 9 pages of information to look at.
http://www.medicinenet.com/edema/article.htm