I do think your problem is coming from not having all your squares cut the same size and also make sure your seams are all 1/4" If both the cutting and sewing are not accurate this can make your quilt off quite a bit..... also when you sew your rows together remember sew one row from top to bottom and then sew the next row on starting from the bottom to the top..... if you don't do that your quilt can also become out of square and looking at your quilt I think that might be your problem........ good luck... keep us posted.
ScrapQuilter