Sorry for invading the heli forums...
As I said to Whirly over the weekend, yes a cold can really knock you and you don't realise. I had quite a nasty cold/tonsilitis the week before my driving test. There is quite a tricky double roundabout which I couldn't get into my head at all. The only thing I needed to remember was to get straight into the left hand lane, from the middle lane, when going straight on... and I couldn't do it. And I drove straight past a light-controlled crossing and couldn't remember what colour they had been. I cancelled my test, got over the cold, and could drive again.
I've never flown with a cold, as I remember how difficult the driving was, and even flying planks takes a lot more coordination. I can't even begin to imagine how one would hope to hover when feeling muzzy in the head.
Whirly, the worst that's going to happen is you might take longer on the course. You're a good pilot, and you know it. You have the ability to give confidence to people, which is a rare quality, and vital in a good instructor. The natural abilities are there, and the rest will come with time. Stick with it, and go easy on yourself.