Is she interested in me or just using me? Very Serious Case, plz help.?

It doesnt sound like she is using you from what you are saying. Dont question her so much and go with the flow and see how it goes. If she was then she wouldnt have come to your door to tell you that she wasnt intersted in the other person. It seems like she likes you but you just arent getting the hint and she doesnt think that you like her back.