What screams out at me from this thread, rose, is that you are partly taken aback and quite frightened by what the nursery worker was suggesting, but also outraged that she has 'slighted' your child in this way. It may not be how you consciously feel, but it comes across very strongly here.
It does seem that the nursery have handled this badly.
But it also seems that you are being overly defensive. There is no need to be. People don't bandy about terms like 'SEN' for fun. Any concerns raised or support/assessment offered will only be in the long term interests of your child.
And do not worry about your child being labelled 'autistic'. I had to beg, scream, shout, cry and demand for 3 tears to have my child diagnosed, so it is completely improbable that the opinion of one nursery worker is going to end up with your child being diagnosed behind your back!
There is no way a nursery nurse could instigate an assessment into autism, btw. Not a proper one. Who has she referred you to (the forms?).
I know it is hard to keep perspective when it is your own baby, but please don't feel under attack on this thread. I am sure nobody wants to make you feel bad, and nobody with an autistic child would dream of suggesting your child is or isn't autistic, either way, because we all know that it isn't that simple.
FYI, just to clear something up in general on this thread - my (Asperger's) DS is in the top 1% for verbal reasoning and 0.4% for non-verbal reasoning in his age group, he was reading fluently at 3 yrs old, he is highly imaginative, has a great sense of humour, is very popular, doesn't display any strikingly strange or obsessive behaviours looks 'normal' to all intents and purposes. His behaviour only really started to hint at a problem at nursery and got progressively worse in school (inability to join in group activities or take instruction from teacher, finding the social constraints of school very stressful). being diagnosed has been nothing but a GOOD thing. He gets the understanding and support he needs.
ASC comes in all sorts of shapes and guises.