There's a simple science behind what happens to a person who changes.
When a vampire decides to change someone they drink a large amount of that persons blood. In exchange they feed a large amount of their blood to the human. The vampire blood over powers what is left of the human blood. In order for the transformation to take effect the human must die.
The human blood quickly dies out, but the vampire blood is already dead and is able to exist beyond death. This reanimates the person, consuming their body and taking over in place of the human blood.
When the new vampire awakens there is a hunger deep within them. The change took time, so their body is starving. Blood is the key. The vampire does not breath and therefore does not take oxygen into their lungs which would transport to the blood and through the body, keeping the body alive.
Without the oxygen the body becomes weak and hunger is a pain that plagues them. Vampires don't need food as their bodies will not be growing or changing beyond the point of death. Oxygen feeds the human through keeping the cells alive. Where food helps to nourish the body of a human allowing for the body to change and grow. Vampires can still consume food to maintain a normal, human looking life. It just leaves the body doing nothing to the vampire.
Vampires get oxygen from the blood of humans. Human blood carries oxygen cells throughout the body. These oxygen cells release from the blood cells and are absorbed into the body. In order for the vampire to get the necessary oxygen to keep the cells of their bodies alive they must take it from the human. They ingest the blood of a human which is carrying the oxygen cells. It is then absorbed into the vampires body, keeping the body alive.