When she has 'that time of the month (period)' she has a whole week of flushing and rushing hormones through her body, that's why women's mood can swing from one end to another in a short amount of time.
Anyway I'm not gonna make you tired with biological jargon I'm just gonna use a few words.
The whole process of period is devided in 3 cycles, let's call that beginning, middle and ending (in biology they call it different).
Firstly it all begins a few days before her period [beginning], where she has a very good emotions running through her, that's why during those first days women are a lot more pruductive, they have better relationships with their friends and they don't want to get in each other's hair, they are also a lot more sensitive to touch, so these early days are good for sex.
The second stage [middle] is when 'ovulation' starts it takes about 3 days and
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the ultimate irony: most chicks i've known are very horny on their periods.
during that time the women is at its peak of horniness, so she'll be more open to sex during those 3 days and she'll enjoy more (stronger orgasms [hormones again 
]).And the third cycle (enidng) is a few days after her period (level of good hormones decreases drasticly) therefore they are more fiesty, they like to fight, they are in everyone's face and they'll decline sex more than usual cause their body is not in that 'function' so to speak.
When all 3 cycles end her hormones levels return back to normal for a month again
I hope I haven't complicated this too much, this is really a short version of it, for anything more specific just google it.
Hope it helps
Cheers
