Swelling in your feet can have many different causes. In a young women like you without any other heath issues, your swelling probably caused by very mild venous insufficiency. This is basically when blood that is returning from your legs going towards the heart gets stuck in the leg veins and has a hard time traveling against gravity. This is seen often in people who have jobs where they are standing for very long periods of time without a break (i.e. a cashier). The end result is that fluid from your blood seeps out from the veins into your legs. Putting your feet up drains the blood from you leg veins and draws the fluid out into your blood stream. There are other causes of lower extremity swelling including heart failure, kidney failure, and liver failure. You don't seem to have any other symptoms that would suggest that these are possible.
I suggest that you schedule an appointment with your primary care physician
. He or she can take a look at your legs, may do some blood tests, and will make sure that it is not caused by anything more concerning. If all these bad things are ruled out, then you will benefit from wearing tight stockings or TED hose which helps prevent blood from pooling in your legs.