Double check your seams, it looks tome as if they might be off a little. And, if the seams check out, Starch your block ,pin it to something that will hold it square, pin all around, then let it dry on its own. Sometimes that will shrink a stretched edge. What the others are saying about starching the fabric really does make a difference, I just found that out recently. You have to be really careful with bias edges, no pulling or pushing when sewing and pressing. Just guide the fabric, let the machine pull it through.Don't give up, the start is just beautiful, you chose fabric really well.