I am told that the resulting headache is a signal that you are activating parts of the brain that were idle, so to speak (or not previously activated).
If the headaches are bothersome, you need to ease back and proceed more gradually. Take more breaks between the meditations or don't stay with it as long. Eventually, you will be able to proceed or go longer.
The dizziness is due to your ability to perceive things in other than our usual three dimensions.