Just checked the website and it just says:
Pregnancy
You should not give blood if you are pregnant or you are a woman who has had a baby in the last 6 months.
Not very helpful as it doesn't say what the problem is...
I know I should have been more careful re timing but a) was not actively TTC again yet following a mc in March, and b) I did test the day before just in case even tho I had no reason to suppose I might be pg symptom-wise - yes I do know how it happens! 
I did feel pretty drained for the next couple of days so hopefully that's why they advise against it. Anyone know anything about this?
Thanks