Ok right now I was sitting down and my stomach shook contantly for like 20 seconds is my baby okay what does that mean? I am 38 weeks pregnant?

Probably just fine. This is a really common thing. It almost feels like your baby is having seizures. That is really unlikely. Usually, it is a burst of movement as your baby finds a comfortable position. As long as your baby is still moving well, there is little reason to worry. If your baby isn't moving at all or is moving a lot less, you should be seen by your doctor.
Don't worry. Don't worry; it could have been anything from your baby moving to braxton-hicks contractions...The uterus gearing up for actual labor. Of course labor is a possibility. Keep a watch on things...