I don’t know how it is in other countries, but in Finland the recommendation for donating blood is for women no more than 2-3 times a year and fo men 3-4. Young women are also offered iron supplements. Before each donation they check hemoglobin but that doesn’t tell much about ferritin stores. And according to that study linked above it takes a long time for ferritin stores to fill back especially if they were low already before donating. So even if the hemoglobin is still above the recommended level, donating every 3-4 months can really bring those ferritin levels down.
I think it’s not that strange that you didn’t have those symptons before as it could be that just was the first time your levels went so low that it actually affected how you felt and gave the symptons. And compared to other ways of losing iron the blood donation makes a rapid change.