For those of you feeling nauseous and missing a period and testing negative, you're most likely pregnant.

It takes a while for the HCG to build up in your body. HCG is detected in the blood before it is detected in the urine; this is why many women test negative on home pregnancy tests, but find out they are in fact pregnant by blood tests administered by their doctor. It is possible to "feel pregnant" before your pregnancy is confirmed. Your body starts preparing for the baby from the moment of conception. HCG isn't the only hormone that causes pregnancy symptoms; there are other hormones (estrogen & progesterone) that increase with pregnancy. The uterus expanding is another cause of pregnancy symptoms.

So yes, it is possible to have pregnancy symptoms (like morning sickness) before HCG levels are high enough to be detected due to everything else going on in your body.