A girl flirted with me, then suddenly stopped. is this normal?

Last year, there was a girl in my class. She isn't particularly pretty, but she was quite funny, and was into the same video games as me. So, I decided to give her a try. I had previously asked out three girls, who had all rejected me, so I decided on a different approach. I tried to be as friendly as possible when I spoke to her, made sure to crack a joke when she was near (she was always the one laughing the loudest). Then suddenly, she started talking to me, which is a first for me. She would make attempts to make conversation with me any chance she got. One time I was walking past her, and she stuck her hand out and ran her hand down my chest. She said my name, and I asked "Yeah?". That was when she looked up at me and said "Oh, never mind...". I was starting to feel pretty good about her liking me, but she suddenly started ignoring me, and talking to this other guy in our class. This other guy is semi-funny (half the time he makes jokes about things no one else knows about... yeah...) and is pretty much average looking. I was very confused and upset. Is this normal? Has this ever happened to anyone else? Also, girls, what do you think was going on in her head? Thank you for your patience in reading my story.