When I was heavier (and sedentary) I was a regular donor. Never had a problem at all. Now, however, I'm down to once a year, right around New Years day. I've found that it takes at least a week to get my usual stamina/energy levels back, now that I'm lean and fit. Probably doesn't help that my blood pressure and pulse are barely high enough to qualify...

Have you consider plasma donation as an alternative? Keeping your RBCs and giving the other stuff alleviates a lot of issues, from what I hear. (With my blood type, the donor center insists on whole blood only from me).