Faint is the action of last resort. It is the result of total shut down and para-sympathetic activation.
If this is a response you have that seems to come about at random times, you might think about when you first remember this happening to you. Interestingly the most frequent experiences of this response I have come across are from medical trauma.
Think of yourself as a child. You are surrounded by adults, and nobody is telling you what is happening. You are scared, being told to be still. You can’t run. You are not allowed to fight. Your nervous system maxes out and your body starts to go numb and you start to become immobile and eventually faint. I think this is why so many adults faint at the sight of a needle and feel fear in medical settings. Medical trauma is a real thing.
Working with an attachment-focused, trauma-informed therapist is the best way I know of healing this trauma response.
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Reference: The freeze-flight-fight-fright-flag-faint defense cascade (Reproduced with permission from Zeitschrift fuer Psychologie/Journal of Psychology, Vol. 218(2):109–127, #2010 Hogrefe Publishing