My 21 week old GR puppy has hardly eaten anything over the past two days; she ate less than a third of her meals yesterday but nothing at all today.  Her poo was a bit runny yesterday but today was back to normal.

I've taken her for a couple of walks today and she was her usual happy self - even in the frost this morning. 

She's lost 6 teeth in as many days (I found 4 on the floor this morning!), so could it be teething that's causing her to be a bit more subdued indoors and not eating?  Surely it's too early for her hormones to be acting up?

Thought I'd try on here first for advice - if you think its necessary, I'll take her to the vet tomorrow,

I would think it is her teeth, But if you are at all worried then get her seen by the vet. Dogs gums get really sore when teething, not always affecting all dogs. But I have found that 'Teetha' powders made by nelsons (can get them from boots or most chemists) work well. They are a homoeopathic medicine used on babies. I have used this with my pup and it helped alot as he didn't have the urge to chew everything insight either :-)
Em

Mine didn't lose their appetite, but I've heard it's quite common. Have you tried soaking the food a bit if she's on dry food, just to make it easier for her to chew? :-)

Thanks to you both - I've tried her meals with and without gravy but still no appetite.  She's even dis-interested in her 10pm Bonio treat, which she usually wakes up for! She just threw it up in the air, pounced on it and then took it to bed. 

It MUST be her teeth - I've just given her some creamed rice pudding, scrambled egg and 2 hot dog sausages, and she's emptied her bowl.  I can go to bed now, knowing that she's eaten at last.  Why didn't I think of feeding her this before???!!