Before worrying about taking supplements, rather implement the following when donating blood.
I did so after being rejected about 5 donations ago, for the first time - have now donated 61 times - and the nurse's where I go suggested I wait for up to a month EXTRA before donating. Therefore, at most I donate 4 times a year - for eg. Jan, Apr, July, Oct or you could make it Feb, May, Aug, Nov.
You're probably trying to donate 5 or 6 times a year, and your blood at your (our ages) hasn't had the time necessary to replenish. Also, ensure enough sleep especially few days before donating.
Coupled with this, as taken from a 1-page article of the SANBS's publication, a few issues ago:
What causes iron deficiency?
(one of the answers): Frequent blood donation
What are the symptoms of iron deficiency?
... include chronic fatigue, weakness, shortness of breath, poor concentration, a rapid heatbeat, hair loss ....
What are good sources of iron?
..best source is liver
Good sources of heme iron ... oysters, seafood, ... beef, mutton, ...... poultry, fish.
Good sources of non-heme iron... egg yolk, dried fruit, dark molasses, whole grains, cereals, bread, nuts & enriched breakfast cereals.
Vitamin C increases iron absorption :
Good sources of Vitamin C inclued oranges, grapefruit, tangerines, tomatoes, broccoli, cauliflower, strawberries, peppers, ... pineapples, Brussels' sprouts, peas.
What factors reduce iron absorption (ie. avoid these as far as possible) :
Tannin in tea & coffee, excessive fibre intake, phytates (in bran, legumes, soy products), oxalate (in rhubarb, beetroot, spinach, soy), antacids & antibiotic/s.