Swelling at 3 weeks sounds quite early, though it can happen with hormonal changes from pregnancy. The thing is, swelling can also be a sign of other stuff like dehydration, salt intake, or even how much you're moving around, so might be good to mention it to your PD or gynae just to check everything's okay and rule out anything else. In the meantime, try keeping your feet up when you can and drinking plenty of water, as that usually helps a bit.