Very late, but negative tests??

I'm a week late for my period but every pregnancy test I've taken (3) has been negative. I'm only 18 with a normal period normally. I haven't been late for a period in almost a year. I'm having slight cramps, nausea, headaches, I've gained 5lbs, I'm always tired and I just don't know. It FEELS like I could be. I'd be completely fine with being pregnant. I'm just confused with why all the tests are negative and I'm this late? I'm not on birth control at all and we rarely use condoms