Terrier suddenly snapping at people

Question
I have a 15 month old female Westie terrier who in the last few months has suddenly started snapping at people. This first happened in my house when some builders were in - she yapped at them then jumped up and tried snapping at their ankles. In response to this I placed her in her pen and only let her out again when they had gone (she generally sleeps in her pen, and stays in it when I'm out at work). I blamed this on the noise and the fact that they are strangers in the house (I realise terriers can sometimes act like this). She has since done this on 2 further occasions to friends she hadn't met before - on these occasions I again placed her in her pen for about 20min then let her out to sniff them and say hello. She was fine while they were saying hello but as soon as they walked away she did the same again-again I put her in her pen
She had started doing the same to men in the park - I started to keep her on her lead, and used command 'say hello' with a treat. This had seemed to work, but today she chased a woman and snapped at her coat - I managed to chase her away and got her back on her lead
I am unsure what I should do training wise (she has been generally well socialised and this has only started the last few months). She has been spayed in November (and had to go back to theatre for a haemorrhage), and also I had a housemate staying for a few months (a friend that she knew well). She has also had an infected paw this month which has now been treated.
I am planning on probably needing to long term keep her on a lead while out, but I am worried that I will need to do this in the house - I also have a 3 year old niece who is closely supervised with her, and so far there hasn't been a problem-she'll occasionally jump up at toys with her but as shes supervised we can stop her, but I am worried that she will snap.

Answer
Jenny,
It sounds like your terrier is out of control and looking for you to become the boss. She is getting nervous, now that she is older, and doesn't feel like you are protecting her so she is becoming defensive.  Start helping her by teaching her commands, sit stay and down, (especially stay).  Also, teach her to walk at 'heel' on leash beside you.  When people come in make her sit and stay away from the door so they can come in.  Then release her with a word like "release" so she can say hello when everything is calm.
It may be helpful to teach her the command "leave it".  Put her in a sit stay, then you put a treat on the ground, say "leave it"  and then pick up the treat. Tell her "good leave it" and give her a treat with your other hand NOT THE SAME TREAT YOU PICKED UP.  Once she has this down you can use it in other areas like when you are walking the dog and she goes to eat something on the ground that isn't good for her, or when she is snapping at peoples heels.
THE FIRST TIME YOU DO ANY COMMAND, IT IS ONLY FOR HALF A SECOND.  A DOG NEEDS SUCCESSES TO LEARN.  THE FIRST TIME YOU TELL HER "SIT, STAY, IT IS FOR ONE SECOND, THEN SHE GETS PRAISED "GOOD STAY".  THE NEXT TIME IT IS 2 SECONDS.  ONCE SHE DOES IT CORRECTLY, SHE IS DONE TRAINING FOR AT LEAST 2 HOURS BEFORE YOU REPEAT THE REQUEST.
Also, teach your terrier tricks, it is fun for you and her and gives her the attention a terrier needs.
nancy