Haven't got my period - not pregnant.

My period was supposed to come August 5th and still hasn't  (7 days late) I've took multiple pregnancy tests everyday and all negative. What's going on? Has anybody ever has this? 
I'm not on any birth control and I use the withdrawal method with my partner but he always pulls out in time. He cums a lot after he pulls out. Even if I was pregnant.. The tests say I'm not. 

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