It is also due to lack of circulation on your legs. When you are near the end of your pregnancy the baby is putting more pressure on your joints, muscles, tissues, and blood vessels in your legs and hips. Upping water intake and also elevating your feet. I find laying on my side in a cool room helps....the relaxation takes the stress off your hips and legs! I use this time to read a book or parenting magizine! If it continues you should contact your doctor!
Heat, humidity, extra weight, being on them for long periods of time, can cause swollen feet! I'm dealing with this right now too!! Drink plenty of water n keep them elevated whenever possible!!
Like pp said its mostly heat but keep them elivated as much as possible will help.
Heat (probably the biggest factor) the additional weight, diet, water retention, etc etc.