Exact same thing happened to me months ago. I contacted Customer Support and they said that once a quest is completed they can not reset it. So Nadia and I are in a very uncomfortable spot because someone sends her a wedding gift when they hear about it.
Don't feel bad. I got a couple of gifts resulting from the last few conversations with her. Hard to remember which was from when because I did like FIVE meetings with her as her affections builds so fast, but they were both like rank 1 or 2 companion gifts; one Courting and the other Luxury. Why would this matter if you already got her so high? Alt bait!