a lot of young girls (specially of pakistani/desi origin) are low in haemoglobin, and although many may not be anemic per se, may still have haemoglobin below the normal range... thats why fainting may occur more often...
i am anemic, hemoglobin is not just a few points below range, but its WELL out of range,
so thats why never tried to donate blood, as noble as it is and as much as i would have loved to, i don't have enough of it to maintain even my own self healthy...
Ask99, either your haemoglobin is a little low (as its usually the case with girls) or the platelets may have aggregated at the site of blood retrieval, causing clotting, and stopping further flow of blood out... thats my guess...