What were Sakura thinking before the cards appeared
Well, now we know that The Clear cards are actually mad by Sakura's unstable magical power that she created the cards without noticing it. In Ep 21 in the anime, we see the she creates Mirror right after she talk about it, she thought about her 'the mirror' card about some kind of mirror that reveal her true self. From here we are safe to say that the cards is created when she feel like she need it or when she missed that magic power.
From here we could make some theories about what was she thinking and feeling when the cards appeared.