Because someone has told her that she will get fat if she eats sweets!! 

I offered her a sweet and she said "no thank you, I don't want a fat tummy"
who would say this to a flaming 2 year old (who happens to be very slim, in 18-24 month clothes at 2.10 years)
I'm so cross that she has picked this up from somewhere. And what where do I go from here?