Uh.. It hard to explain in my English skill, but I'll try Pinkie's mental state is unstable. She was depressed person. One day, Pinkie get into trouble in dodge ball game with RD's class. RD gets Pinkie off the hook, RD became very special person to pinkie. After that day, Pinkie became cheerful person. And Pinkie stare RD anytime, everywhere. (like stoker ) RD dosen't remember Pinkie, she think this situation so weird. And she find out all the information about Pinkie. but Pinkie doesn't stare RD anymore after one day. RD's mind is full of curiousity and futility. Few days later, RD meet Pinkie holding stacks of paper in hallway. This pic is next situation.
actually, full behind story is so long, because it's my friend's rainbowpie fanfiction (Maybe you can't read it.. it is written in Korean)