There could be a number of reasons for this. However, these are two of the more common ones.
The first, she is trying to slow things down.
She may be worried that the conversation is getting too intense and it may lead to topics or questions that she wouldn’t want to be presented with.
In other words, she may be worried that you may start getting the impression that she is really interested in you if she were to keep the conversation going without any breaks.
A girl who likes to flirt for the fun of it will often find herself in this position. She just wants to flirt and have fun conversations without any desire to become romantic with the guy.
Yet, she will often come across guys who think that there is more to the flirting and soon after a few fire conversations with them, they want to be with her romantically.
She has probably learned that slowing things down for a day often tempers the guy, thereby keeping him from falling for her so quickly.
The second, she may be playing a game with you.
She may have these fire conversations with you and then proceed to take a day to respond unexpectedly because she wants to keep you guessing.
In keeping you guessing, she may be hoping that she will get you to chase her.
By taking these day long breaks, she may be hoping that she gets you so excited and bothered that you can’t help but chase her.
Now, this doesn’t necessarily mean that she wants to be with you romantically.
However, if she is playing this kind of game with you, she is intrigued by you to an extent and may want to discover whether her intrigue can become so intense that it compels her to want to be with you.