Believe her first
"Everyone gets period pain" is one of the most damaging things a teen can hear. Some period pain is normal; pain that stops her doing normal things is not.
Start with: "That sounds really hard. Let's figure out what actually helps, and if we need to see someone."
When to escalate
If she's regularly missing school, curled up unable to move, or the pain wakes her at night — that's the threshold for a GP conversation, not "just period pain".
Heavy bleeding (flooding through clothes, changing every 1–2 hours, big clots) also warrants a GP visit. Iron levels are worth checking.
What to ask the GP for
• A full history and examination (age-appropriate).
• Blood tests: full blood count, ferritin, thyroid.
• A discussion of options — tranexamic acid for heavy bleeding, mefenamic acid for pain, and if appropriate, hormonal options.
• An onward referral to paediatric gynaecology if symptoms don't settle. Endometriosis and adenomyosis can start in the teens.