What Causes Delayed Periods?

I am afraid I might be pregnant. I have taken 3 pregnancy tests (one of them was at a pregnancy clinic) all of which say that I am not pregnant. However my period is a month late. Me and my fianc�© are always very careful. When I went to the pregnancy clinic they said I would be 7 weeks pregnant if I was (although their test said I wasn't). I have been really stressing out over it. I am not overly tired, haven't had any increase/decrease in appetite, and no nausea. The only real symptoms I can think I have are the lack of period and urination (though I drink a lot of soda during the day). What do you think?