Most likely you are stretching the edges of your football shapes as you work with them. These will be mostly on the bias, which is much more prone to stretching than straight-of-grain cuts.
Did you prewash your fabric? If so, did you starch the fabric before cutting? Starching adds stability to the fabric so bias edges are less likely to stretch. Typically you want to starch before cutting the shapes.
If your shapes are already cut, try spray starching the shapes and iron them dry. (Iron by pressing down. Do not iron back and forth, as you can distort the fabric by doing that.) I would give each side of each shape at least one coat of spray starch and allow that to dry before sewing.