This is a very common problem in the latter part of pregnancy. It is caused by the weight of your baby pressing on your veins causing a reduction in the blood flow from your legs to your heart.
To help relieve the swelling try the following
Put your feet up as often as you can.
When you put your feet up exercise them.
Cut down on your salt intake
Put the bottom of your bed on blocks or put a pillow under the mattress so your feet are higher than the rest of your body.