Hey, put us together & we'd be balanced! For me, it was my left foot. Turned out to be nothing in my case (though the advice to be concerned about a possible clot is accurate).
Women in my family tend to have poor circulation, especially when pregnant though no one else had it manifest in one swollen foot like I did. I think it was a combination of my terrible Irish circulation and a little one sitting on my sciatic nerve and whatever veins were right around there.
It's years later and my left foot still tends to swell more than my right. Watch out for your salt intake (did you know ice cream and tomatoes both have lots of salt??), drink a ton of water, get some exercise, elevate your feet when you can (higher than your heart -- stretching out your legs on the couch doesn't count unless your head is down too) and maybe use an ice pack if the swelling is really uncomfortable.
Hang in there!