Feeling worse just before venesection

I tend to feel shattered for a week to ten days after each vensection when they are close together e.g. once a fortnight. When they are more spaced out, like 3 weeks or 1 month between, this is not as bad. It takes at least 3 days for me to stop feeling light headed on standing up from sitting and now that I am at maintenance level and it's been a month since my last venesection, I still feel dizzy when I stand up too quickly, perhaps because my iron levels are so low.

I've been having much the same experience. I believe that it is the erythropoiesis cycle. About 9-12 days from the previous phlebotomy, the new red blood cells emerge from the marrow. That's when the macrophages supply them with iron. Initially, that iron is supplied from ferritin stores. But once ferritin is lowered to a certain level - and this likely varies by patient - the organs are scoured for iron. This is the whole theory behind phlebotomy. Hemosiderin in the organs is converted back to ferritin to supply the new emerging RBCs. 

Because I've had sgnificant cardiac iron loading, my phlebotomies occur in a monitored clinical setting. So I have a complete new set of blood work done every two weeks. I think this has afforded some key insights into this process. For me, ferritin levels declined over the course of the first 6 phlebotomies. Then they went up. This confirms that body iron is being accessed. There's nowhere else for it to come from. At some point they will plateau and then go back down again. That's when we reach maintenance levels. Body iron has been depleted.

On these 9-12 day 'anniversaries', I've had a few AFIB events, have felt extremely fatigued, and occasionally have felt rather like my internal organs are being passed through a cheese grader.

It's not in your mind. It's the physical cycle of the extraordinary steps the body takes to try to achieve homeostasis.

I found I got 13 days of relief from fatigue and pain after a venesection, then it descended on me again.  I look forward to my venesections to get that toxic stuff out of me and get those 13 days of relief.  I am on maintenance venesections of every 3 months.