If you were using steam or spraying with water, I would think that would cause the shrinking. I ALWAYS preshrink everything except in the case where directions say otherwise like in the case below.
BTW, did you know that fabric after washing can shrink more in one direction than the other? I had this happen with a Kona cotton solid black. I cut 9" squares for a 7" square finished block for appliquing using a special applique fiber that requires prewashing the applique pieces but not the background. When washing the background shrinks and pulls the machine applique stitches towards the underside of the applique, so it looks like hand applique. Then the block is cut down to size; hence the oversized square cut at the start.
The blocks were square after sewing the applique but before washing. After washing the blocks were not square. I would have to check my blocks again, but I think it did the shrinking on the cross grain.
Advance Warning...I did preshrink the fabric in a kit which included a panel. The panel had blocks to cut apart. The dimensions were off on the blocks after washing, and I had a lot of work to adapt the directions for construction.