Doesn't sound like negging to me. You were communicating (politely and honestly) that you weren't interested. Negging generally involves being impolite (or, at the very least passive-aggressive) and pretending not to be entirely interested, so as to make the other party (usually a woman) desperate for your approval. Sounds like you came up with a good excuse in the moment to buffer the bruise to her ego. If she'd genuinely been hurt and upset about it, I don't think she'd have been so flirty and playful with you afterwards, either; she'd have simply gotten her distance. All in all, sounds like no harm, no foul.