What Causes Swelling In Feet And Ankles?

My wife is 28 years old, 170 lbs., 5'5". No medical history to report really. Her foot and ankles are swollen, but she didnt do anything to them. She was just in Las Vegas where it was 100+ degrees, and she just flew back to Philly last night for 6 hours on a plane. They've been swollen for almost 24 hours. What could it be?