Bias edges, don't you just hate them? If the borders are the right measurement and it's the quilt itself that is stretching, I'm puzzled along with you. You could take it off the frame, lay it out on a clean sheet and spritz it with water and block it into approx square and let it dry. I think though that when you reload it it will just stretch again? All I can think of is doing the best you can to square it as you sew and quilt the heck out of it. If the border is wide enough you could maybe trim some of it to square it up when it's done. Maybe someone else can give you some more ideas. Good luck!