Happened to me once after my first few donations. First couple I was great then suddenly my 3rd or 4th, the next day I hit rock-bottom. It was awful. I read after 3 donations in a year a male's iron could get low. So I checked it and my ferritin level was like 7. I think normal low is 35 or so. I immediately started iron supplements 18 mg per day. Maybe placebo effect but I instantly felt better. I haven't had a bad day after since. However, as of recent, a couple years in, my hemoglobin started climbing every blood donation...16, 17, 18. I started reading about others getting rejected at 20. I had my iron checked. Ferritin was low but my Total Iron was high 190 (180 high normal). I clued in that this was probably why my Hemoglobin was high as well. As well as many others' who helpfully posted in past to be careful about iron supplements raising hemoglobin too high for blood donation. I laid off the supplementation for 2 months and on my last blood donation hemoglobin was lower at 17 point something. I'm no doctor full disclaimer and diagnosing anemia or low iron seems more complicated than just one number as there are a few. Total Iron, Saturation, Binding Capacity, Ferritin, etc. Also too much iron will poison you. So go easy. They do recommend supplementation if you go past 3 donations a year (or something like that, don't quote me, do your research). Just be careful not to use too much. Keep us posted if you supplement if it also makes you feel better. Could also be lower RBC, etc. I drank coffee after one donation and it was like amplified but I haven't been able to repeat that any donation since. Get rest and feel better.