So sorry this happened to you.
my guess is you did not cut the boarder on the straight of grain. A cross wise cut on the WOF will stretch when stitching or when pressing.
How bad is the stretch?
1) If it is a little stretch, can you "ease" it back to square with no puckers or pleats?
2) If it is a lot of stretch, can you make a lovely gathered amount to square it up?
3) If it is a medium amount of stretch, (can't ease & can't gather nicely), how about gathering what it is and covering the gathers with prairie points that point into the boarder with the large part of the triangle tucked into the blinding essentially covering the gathers,
4) Do you have enough boarder fabric to replace it and cutting on the straight of grain?