I had my hep. B shot recently. Tomorrow is the 18th day, since then, and I have a paper that says I might have to wait 28 days before donating blood. However, considering I m a woman, at that time of the month, wouldn t that eliminate some of the vaccination? I really want to donate blood. (The Red Cross gave me a sticker the last time I donated, saying I could donate next on the 7th, aka, last Monday.)