I may not give blood again...

I gave blood this morning in the company blood drive. But this afternoon, I was feeling really strange, kind of lightheaded, and really warm and sweaty. I had to go home from work early, and cancel my pet-sitting job tonight (it requires a long drive, and I wasn't feeling up to it). I'm home now, drinking some water, and feeling a little better. But it will still be nice to rest, relax, try to build up my strength again. I'm kind of bummed, because I like to donate blood - makes me feel like I'm making a difference - but I don't want to end up with "side effects" like this...