Why do some women get more cuddly when on their period?

My new girlfriend is not the most affectionate person nor am I but for some reason reason she is only cuddly when on per period she will want to be held, sleep in my arms and want back rubs which I don't mind but I find it the opposite of my past relationships. My ex would want to be alone when on her period and and really cuddly every other time.
Why do some women get more cuddly when on their period?
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