Under normal circumstances, the blood circulates around the body, reaching the legs, the periphery of our circulation via the arteries.
In order to get back to the heart having delivered its oxygen to the tissues, the blood is aided by the natural pumping action of the muscles in our legs and the one-way non-return valves that are built into our veins, allowing blood to flow only in the direction of the heart.
When we are in hot or humid climates, our peripheral circulation opens up in order to assist heat loss from the body and control temperature preventing over heating.
We also tend to perspire more heavily for the same reasons and can become salt depleted as a result.
The loss of salt from the circulation can reduce the ability of the veins to draw blood back in from the tissues, leaving the fluid in the limbs causing them to swell.
It may be because your body is trying to retain water and you can avoid this by drinking a lot of water. The body does this so as to keep your organs operating when it is humid leading to water retention in the limbs. It is also advised to take in less salty food and do cardio exercises during the humid period.