Swollen leg in pregnancy especially in the third trimester is very normal. It is caused by the increase of bodily fluids in the body, the water, blood, etc all increase in volume. Also there is the weight of the uterus with a growing baby which can also slow down blood flow from the legs back to the heart.
Sometimes other factors like standing for too long, not drinking enough water, amount of caffeine consumed, a none balance in diet and a hot weather can also contribute to it. What you can do is keep your legs elevated on a higher surface whenever you sit. All the best mama.

It should be discussed with the obstetrician at antenatal care.