I was 12. I was in so much agony my parents nearly called an ambulance overnight for what they thought was appendicitis. But Mum had an inkling and the next morning sure enough, there it was.
I've never been regular though... well, they've always come monthly, but they're reasonably heavy and could arrive anywhere 4 or 5 days either side of the '28th day', which is annoying. Made far worse since I came off the pill (which I was on for 8 months and never want to touch again my life, it was absolute hell). Now every month I think I'm pregnant until it arrives... talk about making a nasty experience worse.
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We are a fever, we are a fever, we ain't born typical...