Thank you for donating blood -- our supply is always in need of generous donors like you! Donating is probably not the specific cause of your period being late. Although because it is true that stress (physical or emotional) can definitely lead to a late period, I would say that it could be a contributing factor. Other things to consider are changes in sleep, diet, physical exercise, infections, other hormone imbalances, and (like you alluded to) pregnancy. It is not too unusual to have a late period once in a while, but if it becomes a pattern, your PMS symptoms continue or you are more than a few weeks late, make an appointment to discuss with your healthcare provider.
I think missing your period after donating blood is more anecdotal than factual. The fluid that you lose during your period is the lining of your uterus that has been thickening but is not needed, so it really doesn’t have anything to do with any blood you may donate. And that’s why we have medical experts here, so be sure to read what Sandy has to say.
I have never experienced this before! It seemed unlikely to me that losing a little bit of the blood pumping around in your body could affect your uterine lining, which is a little different. But since I’m not a professional, I’ll defer to what our expert has to say on the matter!