Regular Plasma Donator-donated last week and almost fainted

So I donate plasma to make a little extra cash when I need it. I used to go a lot before but have kind of cut back because I haven't needed the money. Plasma for those that don't know what I'm talking about is where they take out about a cup of blood one at a time and they remove the plasma from the blood and put your red cells back in and that happens about 9-14 time throughout the process. Most of what they take out is water but they take about a half a gallon of liquid in total once it complete. They do give you saline at the end to help make up for what is lost.

I went last week for the first time in about 3 months I felt fine during and after. But later on that night I felt like I was going to faint even while I was sitting down and ended up having to lay down to not black out. I'm eating 1300 calories a day and I drink a little under a gallon of water per day and I'm doing P90X. I didn't work out on the day that I donated. Maybe just everything together was too much? Any tips would be great... Thanks!