Right after eating a meal and drinking all of water i felt faint and almost passed out. also felt like i might vomit. what would cause it?

Gastroenteritis: A large meal with a large amount of fluid intake may induce nausea due to a delayed emptying of the stomach of the food/fluid or possibly from a viral or food borne illness. If it is something that only occurred once, I would not be worried about it but I would note what food you ate and fluid you drank. Anti-nausea medications can help if nausea is persistent.