Period is a week late

My period is a week late! I am 15 and sexually active. I've never had unprotected sex. I am doing my mock GCSEs at the moment and I am rather stressed. But my period is 7 days late! I'm that stressed I've taken 3 cheap pregnancy tests from Boots and they've all come out negative, but I'm not convinced that I've just skipped it. Can somebody please give me some advice or put my mind at ease😓