@Linkat,
Well i will try to explain this on non medical terms.
The central nervous systems controls everything on our body, from our figers typing our ideas, to the simple act of breathing. Breathing as well as hearth beat are completely automatical. As well blood preassure, gut movement etc. Ths are controllesd by something called Auntonomous nervous system (ANS). The ANS regulates every aspect of our metabolism to keep everything working leaving the brain without worry. So it does not needs the brain to work (that is why when a person has brain death the hearth keeps beating).
ANS uses a dual system for control that coordinately work on opposites. Sympatic and Parasympatic systems. Example a bronchi stimulated by Sympatic system will dilate, but with parasympatic stimuli it will constrict. Sympatic nerves are scaterred and depend a lot of the periferal nervous system. While parasympatic system has his own particular nerve called Vagal nerve (that is way it is called also Vagal system)
On the blood pressure sympatic system increases pressure and the parasympatic system will lower pressure. When we suddenly stand up gravity pulls all the blood from our head and torso to the feet, but both our autonomic systems quickly react balancing the pressure one causing vasoconstriction and the other causin vasodilation. If the regulation doesn´t kicks in quickly we can feel a small vertigo or even fall unconscious.
The Vasovvagal reaction is a malfunction of this regulation system. The parasympatic system overreacts to an specific trigger. It is not an allergic reaction, and it can happen once and never come back. The dilation of vascular vessel can be so huge and fast to cause a syncope. Or slow and gradual to ressemble drowsiness.
It can be caused by pushing while being constipated, or by an inyection, or by the sight of blood.
So that is what i think that happened to you. Also to Romeo