So i ate a good breakfast and drank lots of water before i donated plasma today but i donated really slow about an hour later i almost passed out why?

Poss Fluid Depletion: Removing plasma can leave you fluid depleted & lower Blood Pressure. You need to replace fluid afterwards. You may have been "Vagal". The Vagal nerve is a cranial nerve that monitors your temperature and your BP as well as your cardiac output. It fires because evolution found that you are better off lying down under such conditions. Vagal stimulation can cause fainting. Important to replace fluid