your fabrics may have stretched a little as you stitched. it doesn't take much, and this has happened to just about all of us at least once.
after you take it off, press it flat again. starch the bejeezers out of it.
find the middle of the border strip and match it up to the middle of that side of the quilt. pin from the middle to the right end, and then again from the middle to the left end. space your pins about an inch apart.
if you have a walking foot, use that when you re-stitch on the machine. if you don't have one, just sew slowly and keep a careful eye on things as you stitch down the seam. that way, you'll notice if the border strip starts to puff, ruffle, or do anything except stay flat against the quilt. if you see it, stop. leave the needle down, but raise the presser foot. smooth it back into shape and then start sewing again. you may have to do this a few times.
take your time, and you'll see it goes much better this time. :wink: