Just guessing but it sounds as if your edge may be uneven, thickness-wise. I would try moving the topstitching over, so that the presser foot rests upon a thinner, more even thickness, rather than the edge. If you find that moves it too far away, consider using a zipper (or similar) presser foot - one which rests the foot on only one side (the thinner side) of the seam. You can then move only your needle closer to the edge (still keeping it on "level" thickness). Go slowly, when you stitch.
It may be too late for this . . . but when turning thick seams, it helps to first grade them. Make your initial seam at least 1/2" - 5/8" and then trim the various layers, to different widths (like tiers). This creates more of a slope, than a bump. Here's a better explanation -
http://historicalsewing.com/tutorial...eam-allowances
What would be easier, is to take the layers apart and instead of right-sides-together-and-turning, just sandwich them, as you would a quilt. Then bind. For binding, I would use a single layer of the PUL, folding over and machine stitching it on. This way would result in sewing over only one layer (each) of the thicker fabrics, with no crazy bumps.
