You are probably just making yourself sick. A woman can actually convince herself that she is pregnant and develop symptoms of pregnancy. Also, on a normal basis, if you stress about it, your period can be late (or not come) and you can make yourself sick.
So, go to the clinic. If you have a family planning in your area, go there. It is free if you are under 18 and they do not need parental consent. You can get an accurate test to make sure you are not pregnant and you can look into going on the pill or something. Plus, they will give you free condoms that you need to use to protect yourself, not just from pregnancy, but disease as well.
Everyone does make mistakes, and I sincerely hope this one does not cause you eighteen plus years of work. Let us know how you are and good luck to you!