To be perfectly honest, it might not have been your speech that seemingly changed her interest. It could have been any multitude of things down to as simple and incomprehensible as a difference in vibes or wave lengths that she felt was mismatched. People are of course all individually very complex creatures in their interests and desires. Or even more simple: maybe her initial smiling aura was purely of friendly intent and not flirtatious, but she might have sensed your flirtatious interest in her and she wasn't interested or receptive to that so she decided to nip it in the bud by minimizing conversation to try to passively tell you she's not into that kind of relationship.