quilting question
I made a table topper sized quilt with 3 1/2" squares. They finished at 3". I stretched the back and taped it down. Followed by cotton batting and a well pressed top. The seams were straight at this point. I pinned in the center of each block. Which meant, pins 3" apart. Seams still straight. During the quilting, on my Pfaff home sewing machine something happened. Several places "pushed". The seams were no longer straight after stitching in the ditch. What caused this ?