DD is 6. She is a confident, bright, happy little girl and DH and I are very proud of her. We use positive praise and she generally has good behaviour.
I play what I call the hands up game when in the car with her, eg put your hand up if you're clever, beautiful, pain in the bum
etc.
We hadn't played this for a while until a couple of weeks ago, and I said put your hand up if you are beautiful. DD didn't put her hand up. I asked her why and she shrugged and said she isn't beautiful. I didn't make an issue of it, thinking she was just in a strange mood.
Tonight, DH was settling her in bed, casually said "go to sleep pretty girl" and she replied "I am not pretty, not one little bit." 
I asked her why she thinks like that, and she got a little upset, said she just isn't pretty. We gave her lots of cuddles and reassurance and she settled ok.
I can't understand why she is thinking this way. I asked her if anyone has said something to her and she says not, but I am not convinced.
Has anyone else come across this please? How best to help her?