Well, there is always the risk of a false negative, but the risk is low. Periods can sometimes skip or come late, but it's best not to bet on this. If you don't get your period soon, you should see a doctor. It may not mean that you're pregnant. You did mention that you are sexually active and that you had some unprotected intercourse recently (the hot tub incident), so it can be indicative of an infection, as well.
Obviously, I can't diagnose you over the internet, and I'm not a doctor yet, anyways, so that would be irresponsible. However, if you don't get your period soon, make an appointment with your doctor to get checked out. And do be sure to be honest with your doctor about your sexual history. We're not there to judge, we're there to help you stay healthy.
------------------
-Mike
Certified Mad Scientist
"90% of people are
caused by accident."
__________________
-Mike
Certified Mad Scientist
"90% of people are
caused by accident."
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pregnancy tests 
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