The problem might be in your pressing or even the thread. Try 50 wt for piecing. Or it might've been in the cutting. All my piecing tries to "shrink", so I use a scant 1/4" seam allowance when I can. A test to see what you have to do to get accurate piecing on a given combo of fabric, thread, sewing machine, pressing is to cut three strips 1.5" wide. Piece them together and press the center strip should be exactly 1". If not, try troubleshooting from there. I still struggle with this after 20+ years of quiltmaking, especially when switching machines. One big factor for me was thread. Switching to 50 wt Aurifil or Masterpiece by Superior Threads has helped a lot.