How long until im sure I'm not pregnant?

Lauren
I have just missed a period, literally just missed it. ( would've been my last day today) been having unprotected sex with my long term partner for a few months, my nipples are sore and tender, have been for the last 5 days or so, but two pregnancy test have come out negative? 
​Should I wait another week?
​I thought the test would be able to detect if I was pregnant by the time I've missed my period? Confused! Help?