There are usually 2 reasons why a border is wavy.
1.) the fabric stretches
2.) instead of measuring the center of the quilt and cut the border before sewing it to the quilt, you just sewed the border on the edge and then trimmed it when you got to the end. (I'm not sure why that happens, but it does.)
You have 2 choices.
1.) you can make darts and quilt it so it blends in.
2.) take it off, measure it trim and re-sew it on. It will be sticky if you have already done the spray basting, but it can be done. If you have enough fabric to make a new border I would probably do that instead of trying to sew the one that is sticky from the spray.
Most of us have had this experience one time or another. It usually happens when I am in in a hurry and think I can get away with the quick way without measuring.