Pranayama is incredibly misunderstood . often abused, and unsafe in the hands of the unknowing, undirected, unaligned.
Students begin a yoga practice with yama and niyama (classically). However the practice these days frequently begins with asana. Either way, asana can be utilized to open up and prepare the body for pranayama. There are two basic preparations. The first is to teach the student to align their bodies so that the energy of prana flows safely in the appropriate channels. The second is to provide opening in the pneumatic system of respiration. This latter component can often be realized through appropriate backbends, opening in the shoulder and hip flexors (and a proper diet to inhibit congestion).
Until the student has alignment is it often best to approach pranayama in the supine pranayama position using gentle pranayama. The ones that I feel are qualifiers (and therefore teach) are Ujjayi, Viloma I, and radiant breath. Beginning students do this following an appropriate asana practice for perhaps 4-5 minutes. As they continue to grow their practice that timing can be increased gradually. If they do not, it is not.
With the particular pranayama you mention is is incredibly common to mistake oxygen deprivation for enlightenment. Getting dizzy and seeing stars and lights is not likely to be Samadhi. However the breath is not without effect and for you it fatigues and accelerates metabolic function. Why? I would not venture to guess.