Looking for a childhood book?

I don't remember a huge amount about this book, but I know it was about this girl in an amusement pArk, all alone on valentine's day. At one point, somewhere near the beginning of the story, the girl has to go to the library and search why we have valentines day. A little bit after that, she sees the walls start to ooze green slime and passes out. Somehow she makes it to the carnival and this boy keeps showing up in random places, like the tunnel of love. That's all I got :) thanks guys.