I have recently acquired a 13 week old female Westie. She is, usually, a lively, happy, bundle of fun. However, this week she has become very aggressive around her food and food treats. She growls and if you don't back off, she really 'goes for you'. She is also a little nervous around strangers outside and other dogs, she tends to bark and back away. I took her to a puppy class last week and she went crazy.  It was her first encounter with other puppies and she was totally overwhelmed. At first she cowered but then she just went hell for leather for every dog that came near her.
She does the usual puppy behaviour, chewing everything and mouthing at you during play, but this is a great worry to me and I realise I need to stop it right now but don't know how! Any ideas??

Firstly she's behaving quite normally.  However, I'm concerned that she became overwhelmed and learnt to drive other puppies away by being agressive.  This should NEVER be allowed to happen and it's better to sheild and protect a pup whose nervous than allow a 'free for all' to develop as pups will soon learn that agression is the way to cope when pushed into a corner

I find the best way to handle a pup whose food agressive at home is to teach them that humans near food are a good thing by adding to the meal as the pup eats and to play 'swopsies' when they have something they shouldn't, by offering a small reward that has a higher value than what the pup already has.

I would also assume that her diet is appropriate in quantity and frequency for her age - ie, she's not hungry....

>It was her first encounter with other puppies and she was totally overwhelmed.

How was she with her little litter siblings, just out of interest? Only I'm surprised that a pup as little as 13 weeks would be so aggressive and it sounds rather like the reaction of a very scared pup.