About a month ago, I was a blood donor. I almost passed out, or so I think. I got real sweaty, naseaus, my chest became tight. I've given blood numerous times before and that has never happen. Ever since then, I have not felt the same. My heart is alway racing, I feel like I'm in a fog, I'm achy, my chest is still tight, I'm clamy, I have a hard time consentrating/focusing. What could this possible be?