My Quora Answer to: "If someone feels like they are about to pass out but don't, and this happens twice in a day under different circumstances, should they see a doctor?
Yes.
As Janet Parker says, there are several causes of near syncope, or the feeling that you are going to pass out. “Syncope” is when you actually do. Falling unexpectedly is no laughing matter, and a doc will help you sort out why in order to prevent hurting yourself.
Common causes include dehydration, electrolyte imbalances, anemia/blood loss, heart arrhythmias, and neurological issues. Even if it feels like a minor occurrence, you want to nip it in the bud if at all possible.