Not feeling good after donating blood?

I have given blood many times before and I've never felt sick after before. I felt fine right after giving, and I did the whole juice and cookies thing after. I'm healthy, young, and well hydrated. About 20 minutes after giving, I almost passed out. I rested and ate and was feeling better, but then I felt light headed again and threw up. What could be causing this? Is it because I just finished my period or what?