Well if the translation is correct, I don't really blame Naruto for thinking she is lying, (which she probably is, Not saying she isn't in love with Naruto but I don't believe she's come to the realisation herself on how much she loves the blonde, more come to the realisation she needs to give up on the fantasy of Genjutsu Sasuke, who only really exists in her mind.) The speech patterns and wording seems like she was trying to convince herself more than Naruto about her feelings, and Naruto picked up on that.
She needs to take a step back and really think about things clearly, at the moment her resolve is of a loyal Konoha nin, a best friend and teamate who she wants to save Naruto from any frther heartache being caused because of her promise. Basically sounds like she's doing what she thinks is right, her feelings did not need to come into play just yet, If she'd just told him the Rookie 11's decision and her feelings about Sasuke it would probably be of the better, maybe leave the door open for them furthering their relationship, but and out and out confession after telling him not to worry about going after Sasuke when his last memory of her was her crying over Sasuke. No one is going to beleive you are being honest, not when you were crying over someone else not even 48 hours ago.
If her feelings are genuine or not who knows, but I don't think her confession was.
Hoo boy we've got a troll.