The vet looked at our little cat today, and also asked a colleague's opinion. They think the awkward walking is due to a reaction to the suture material used to close the spaying incision, as there was some thickening of the muscle in that area. The vet thinks it's probably pulling a bit when she moves, so we should continue to keep an eye on her, but there doesn't appear to be any other reason for concern, which I am very relieved about.