First off, thank you for giving blood! there are too few folk in the U.K who do.
Second, I have been refused a few times because I am Anemic on and off.
Don't worry about it. Some days it is just as simple as not having got a lot of protein the week before and they want to be careful.
I am usually just fine giving blood, but had one occasion where my iron was juuuuust above the required level and I passed out half way though my pint.
Better safe than sorry.