My girlfriend won't sleep in the same bed as me when she's on her period?

I always know when she's on her period because she tells me but that full week she won't sleep in the same bed as me we live together she sleeps in the spare room!
We've been together for a year and a half. It got on the bed once I didn't make a big deal because it's nothing but after that she never slept in the bed on her period again.
I want to know why she won't?
how can I get her to just sleep in the bed?
My girlfriend won't sleep in the same bed as me when she's on her period?
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