well, first of all, the baby is growing more rapidly at this stage, starting to put on weight. Also, could you be retaining water? have you eating a lot of salty foods lately?
you'll probably laugh at how anal this is, but I do weekly measurements, including my calves. today I weighed 1.5 lbs heavier than I did yesterday and my legs felt a bit odd, and sure enough, I measured them and they're a whole inch larger than they were yesterday. when this happens, I cut out salty foods, for a couple of days, try to drink more water, and my weight goes right back down again.