Telling a fib for the greater good

Is it wrong to lie to a nurse?
I'm a regular blood donor and went to donate yesterday. The last time I donated I had a fainting incident several hours afterwards which was entirely my own fault for not taking enough care of myself post-donation.
The nurse had the sort of extra-sensory perception that only nurses have and asked almost immediately if I'd fainted after my last visit. I owned up but said I'd called the helpline and been advised it was probably dehydration.
Now at the time I said it, I believed it was true but quickly realised that I had in fact only gone on the National Blood Service website for advice.
Chose not to reveal this recollection.
And I'm glad I didn't because when the nurse checked in the official 'folder' it said that people who had fainted weren't supposed to donate again.
I'd have been gutted if I'd been banned not least because I'm so close to getting my silver donors card.
Fortunately my fibbing saved the day. Because I'd 'spoken' to a doctor and 'they' hadn't mentioned not donating again the nurse thought I should be OK. Although she did have to fill out some paper work to go on my file "you understand".
Felt a bit naughty but I hadn't intended to fib and I did donate and everything was fine. No fainting. 
The silver card remains in my grasp.