Why haven't I had a period?

Okay, back on December 28 I had sex with somebody I work with after work. We used a condom and I am not on birth control. I never got my period in January. It's Feburary and I still haven't gotten one. I've token three pregnancy test and they all say negative. I'm starting to get worried on why I haven't gotten a period. Any suggestions on why I haven't? 

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