What does it mean if my date was happy until someone else asked her out?

Long story shorter. There is a girl that I really like. I bought her a rose and asked her to the homecoming dance, she accepted the rose, said yes, and hugged me. We were both very happy that day. A day later, someone else bought her a rose and asked her to the homecoming dance too. Of course she didn't accept the invitation, but she took the rose anyways. When I found out about it, I talked to her about it. I told her that I am sad because someone else gave her a rose, and I'm worried that she accepted it. She told me that she didn't accept the invitation and told me not to worry about it. I sensed a change in her mood, she seem sad and quiet, which was unusual. After we talked about it, we both ended up being sad and quiet. She ignored my message and set me offline on Facebook.(I think.)

I think that she likes the other guy more than me and because of me, she can't date the other guy instead.