I'm able to feel a difference between feeding and sucking. When my dd feeds she makes a light "ca ca ca" noise and sometimes, if I listen carefully, I can hear her swallow whereas if she's sucking it actually feels different and she makes no noise. How long does she normally feed vs her df? I know we shouldn't time a bf session but it may help you as well. Keep in mind they may feed slower during a df, at least my dd always did.