Itching inside the vagina during your period is usually from increased moisture and friction, or from a yeast infection that flares up easily around periods. Scratching breaks the skin and can make it worse, or invite a secondary infection.
Next Steps
An OTC antifungal cream like clotrimazole can ease a yeast-type itch. Change your pad often and avoid scented soaps or wipes down there. Please see a gynaecologist in the next day or two if it doesn't settle, since testing can confirm the exact cause.
Health Tips
Avoid scratching further, and go in sooner if you notice unusual discharge, odour, or swelling along with the itching.