It's a common assumption. Especially if they're telling each other "I love you," having sex, and it (a committed monogamous relationship) is what one side wants. She does seem to be giving mixed messages - saying "I love you," saying "I want to be single," etc. But yeah not everyone has the same idea when it comes to dating. Some people are not serious right away, some people are. Honestly, I can't tell from the OP's post if she is a liar or if she is just confused or not communicating properly or what. That's why I suggest being direct in communication.