In my opinion, there is a great possibility that the girl likes you, but she is not sure if you like her. This may have been caused maybe by the way you act or carryout yourself. If you are the type that likes jokes a lot, that maybe a reason why she doesn't believe you. You have to prove to her that you are for real.
A girl that would actually say, 'I don't believe you' after you express your feelings to her is not worth your time. She may want you, she may not.......she's not worth your time because she gave you such an unblunt answer