What could be the reason why i feel like passing out after a blood test?

Vagal response : The most likely explanation is a vasovagal response -- an exaggerated response of your nervous system to the anxiety of having blood drawn, trying to calm things down but working a bit too well. The result is a drop in heart rate and/or blood pressure that makes you feel like passing out. It's not dangerous -- lay down until symptoms pass. To prevent it, keep yourself well hydrated at all times.