How did you measure your borders? Did you just cut a long strip and sew it on, then cut off the rest? Or did you measure the center of the quilt and then cut the border to that measurement? Did you starch the fabric before cutting it? That will help prevent stretching.
If you didn't cut the border to match the quilt, sometimes this will happen. If you did measure it, did you pin it in place and avoid stretching the border?