Why i fainted after the vaccination?

Vasovagal episode: You had what is defined as a syncope. Sometimes unpleasant events/pain during vaccination can cause a drop in blood pressure and your autonomic nervous system reacts and can faint. This can be compounded by dehydration, stress and body position too. Sudden change in position, laying to standing can trigger this too. It is generally safe as long as you do not get hurt. Hope this helps.

Syncope: Do you normally cope poorly with needles or the site of blood? Do not worry, this is a common problem, you probably suffered from "vasovagal syncope" which is a complicated process in which your vagus nerve temporarily malfunctions in response to stress or pain.

Vaso Vagal Suncope: It does happen to some as result of being get stuck with needle No long term effects

Vasovagal: You might have had a vasovagal syncope which is not much to worry about . It can happen with vaccination. https://www.cdc.gov/mmwr/preview/mmwrhtml/mm5717a2.htm