Pranayama cleanses the blood and the respiratory systems as it works to clean the nadis and chakras. During this process many impurities are purged from the physical and internal bodies, and the changes in the flow of energy can result in various sensations, including dizziness, light-headedness, even mild fainting. None of these experiences are dangerous, nor do they last long. Instead, they are signs that the exercises are working. If you are uncomfortable, simply reduce the intensity or duration of the practices, but try to continue so that the impurities will be purged and the bodies will become accustomed to the corrected and improved flow of energy.
You may also find that doing some light exercises, such as walking, swimming, or riding a bicycle, will aid the effectiveness of the practice, since these physical activities improve blood flow and breathing.
It is also important to note that all pranayamas should be done with mindfulness, deep relaxation (not tension), and prayer.
"Do not worry; cultivate the habit of being happy." - Samael Aun Weor