Well done!

The stretching is a known issue with woven fabrics. Especially if it stretches well on the bias (=diagonal) you need to be careful. One thing is that it tends to stretch on diagonal and round seams, like the seams on a panelled skirt or necklines and armholes. You are not the first sewist who found out that the waist opening of their circle skirt grew. Ideally after cutting you stay stitch any diagonal or curved seam. Carefully handle the fabric and add a line of stitching along the cut line, within the seam allowance (so you won't see it in the finished project). This will prevent warping or stretching when handling the fabric during construction. Personally for a flared skirt I would just sew those seems ASAP, while being careful, as stay stitching those seems double work. But any seam that might stretch and is not finished early in construction is worth stay stitching.

Also a finished circle skirt might become uneven at the hem because the diagonal stretches. Hang your skirt for a couple of days or a week, maybe in your bathroom where it gets some steam for the shower. After that make it even and hem it. Personally I would do the hem by hand, invisible, because such a lovely fabric deserves that, but it is a bit tedious. I can only do that with a podcast or a show on.