I'm a regular blood donor, and I've never had this problem before. Why do I feel so weak now?

I donate blood fairly regularly. Not like every 6 - 8 weeks or anything crazy but more frequently than a lot of people. I've been donating since I was 16. My build and diet haven't really varied too much and I've never had any faintness or weak feeling after donating blood before. I'm ovo-lacto vegetarian, so I've had some issues with iron before, but I've been keeping up with it for a while, so that wouldn't be the problem, it was high enough to donate and everything. It's just, it's been over 24 hours since I donated and I still feel weak. I mean, the lady who set me up was kinda rough with the needle, but that would not cause me to be like this for so long after donating. I know it's a little bit paranoid, but rather safe than sorry... Is it normal for people who've never felt weak after donating blood for over 4.5 - 5 years of regular donating to have this problem all out of nowhere?