You might have inadvertently flicked the right stick left or right when choosing your play. This allows you to swap players in your formation (e.g., putting a WR at HB). The control can be a little touchy, though (I've found that sometimes I have to flick the stick three or four times to affect a change, other times, the change happens with one flick).
Also, this change will hold until you change it again from one play to the next, even into your next possession; in other words, it never returns to a default "package" automatically.