If your sewing position is the problem, I would think your leg would fall asleep or get a cramp - not swell up. I'm glad your last physical was fine, but I'd still ask the doctor about it.
Maybe show him/her a photo of how your leg looks after it swells up. Make sure your other leg is in the photo, too, so the doc can compare.
You might also ask someone to look at your sitting position after you've been sewing for a while. Maybe even photograph it, so you can both study it for what you might be doing that leads to the swelling.