If your fabric has any polyester in it, it will shrink when the hot iron touches it. If it was printed on a lower thread count cotton fabric, it may also shrink when the heat touches it. Unfortunately not all fabrics are of the same quality no matter where you buy them or how much you pay. I find feeling them and holding them up to the light helps me decide if it is a quality fabric. If they look or feel "thin" then they probably won't do for your quilt no matter how pretty.