Me And My Girlfriend Are Sexually Active , Both 18.

Me and my girlfriend are sexually active , both 18. Her period has started bleeding 7-8 days earlier than usual , says its lighter than normal , and says she's having back spasms. We are emerald safe with sex and believe there's no possible reason to be worried because I've never ejaculated inside of her. She also normally he back pain during her periods but she says its a different pain. And she said she doesn't see any of the light red and brown spots that I've researched called implantation. I need advice and help on anything you could say we do and what you think is happening