Right, pregnancy signs and symptoms often don't show up for two weeks at the very earliest. Many people don't notice any until a few weeks after that even. Keep in mind that stress can make your stomach hurt and make your period late. So a lot of people mistake stress for pregnancy!
You do not have to be ready for a baby when you become sexually active, but you do have to be ready to handle a possible pregnancy should one occur. That means being ready to raise a child, have an abortion, or put the baby up for adoption. Sometimes the decision needs to be made quickly because women can usually only get abortions during the beginning of the pregnancy.
If you decide you are not ready to handle that possibility or just not ready to handle sex in general, no harm done.

You can always try it again later when you do feel ready. It takes a very mature person to admit that they might not be ready to handle something yet.