Hey hun,
I have had this before, I even got to the point of ringing a Pregnancy Hotline, it turned out that the nausea I was feeling was actually psychological, the lady on the phone reassured me that I wasn't pregnant. She could also tell that I was extremely anxious and petrified about the 'pregnancy' Afterall, I was not pregnant. As soon as you start to concentrate on the pregnancy symptoms, the more likely you will THINK that your pregnant, basically you wouldn't usually count how many times you go to the toilet, but as soon as your anxious about pregnancy you will concentrate on all the symptoms.
My advice, call a hotline for teen pregnancy and explain your position, what symptoms you've had and get a test done by a doctor (or one at home)
You can also take a pill that is called the "Morning After Pill', here in Australia, when I have sex with my boyfriend and we don't use a condom I will take that pill. It is a contraceptive for the times when you forget/break your first contraception. It works very well, but maybe since it was a week ago, wait another week to have a test (the 'Morning After Pill' may not work after a week, it is best to use straight after sex or the morning after.
Hope this helps!!

Don't stress!!
PS sorry its long!!