Easy way to answer this is to reverse roles.
Say you were at school one day, and some girl you know was all about you - I mean she's looking at you with THOSE eyes, touching you, talking to you - basically you're both having a great time, and she's looking really interested.
So you go home that night and think about it, and what it meant, etc. etc...and then there's no way in hell you come back to school the next day and act exactly the same, because things aren't the same, they're progressing, you know?
That's how it probably feels to them. It's not a bad thing, in fact it shows they're thinking about you. Not bad.
But also, just like everyone in the world, women have moods, some days people are happy, some days they aren't.
Also, gotta watch out for when they're on their periods. Sometimes that's pretty nasty mood swings - handle with extreme caution (let's just say I'm speaking from experience

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