Don't corner the conversation
Face-to-face at the kitchen table is high pressure. Side-by-side is low pressure — a car journey, a walk, doing the dishes together. Bring it up as a passing comment, not a Big Talk.
"I noticed the pain-relief pack was low, want me to grab some?" is more useful than "we need to talk about your period".
Offer a choice, not a demand
"If you'd rather text me than talk, that's fine." "If you'd rather ask Auntie/Nan/the school nurse, that's fine too — I just want you to have someone." Being the trusted adult doesn't always mean being the only adult.
When she does open up, don't over-react
The first time she mentions cramps, mood, or a scary flow, the most powerful thing you can say is a calm "okay, tell me more". Not "oh no!", not immediate problem-solving. Just space.
Then, when she's finished, offer one small next step — a hot water bottle, a pharmacy visit, or logging the next cycle together in the app.