Becuase that's what happens to girls. Or you just get turned on by something and you don't know it.
I don't remember where i read this ( i think some website ) but i believe you can have implantation bleeding- something to do with the lining shedding.
For many woman it does, it did for me, but not in all of my pregnancies.
The most obvious is that you miss your period. Your chest may also become tender. You may feel a it ill in the morning or evening. The best way to find out is to go to your doctor for a blood test. But if you can't wait to find out, the pregnancy tests that are sold in local pharmacies like CVS, Walgreens, etc. are inexpensive and more accurate than ever.
Have you always been regular? Sometimes worrying about if you are pregnant can cause enough stress to make you a few days late. (This is coming from someone who was chronically late as a teen). The nausea may be caused from worry, too. As far as the stomach getting hard and feeling heavy, my doctor told me it was from the lining in the uterus building up. On the other hand, home pregnancy tests can be false-negative. Relax, give it a few days, and if you don't start within a week, go to the doctor or family planning clinic. There may be other underlying reasons for you to skip that need to be eliminated.