Should I donate blood again?

I have donated three times. The first time was a last minute thing, I saw it in the paper and thought why not I think I'll do it. I didn't do anything to prepare and I ended up being fine after. The second time I ended up passing out at the canteen and woke up in my juice. Yesterday I donated for my third time and I didn't pass out but I was very dizzy every time I sat up. I think they kept me on the table for anywhere between a half hour to an hour. (I was their last patient because I usually go an hour and a half before it ends) For both the second and third times I ate good and drank extra. A couple of the red cross people were surprised to see me back. I'm wondering if I should do it again. I really want to because it makes me feel good. It is very embarrassing to have such a reaction to it though. What do you think, should I keep doing it? Also, is there anything to do so I don't get so dizzy?