Our 16 month old girl is generally fine towards other dogs but has always seemed to like some more than others. However, she has started to show aggression towards other dogs and in the past 2 days has started 'fights' with 3 other dogs, 2 of them we had never met before the other we have met occassionally. When the 'fights' start she ends up stood on her back legs (the other dogs have done this to) and makes a lot of noise but doesn't seem to bite or even really bare her teeth. However, she is fine with other dogs we have met in the past couple of days. She also bit OH today when he tried to take something off her she should not have. This is really out of character and she has NEVER shown any signs of aggression to any person so this is also a real worry. There was one point where she had broken his skin so it was quite a hard bite. She also gave no warning eg growling that she was going to do this.
I'm worried at the fact she is becoming so unpredictable around other dogs (and possibly people) when before she was always fine and I want to try to solve this whilst it is in the early stages before it gets any worse.
We got another dog 3 months ago so I don't know if this has anything to do with it? They are fine together - took them about a week to sort out the dominance and have been fine since apart from the odd grumble. Also the 'fights' she has started in the past 2 days have all been whilst our other dog was not with us (one was at the vets after her booster, then in PAH then today whilst out but have been doing lead training hence why other dog wasn't there). Could this be a reason? Could her booster have had something to do with it?
Also she finished her season 1 -2 weeks ago - could this be a possible cause?
I would be really grateful if anyone could give any advice as I'm really worried about her behaviour and as I've said really want to get it sorted asap.

Have you considered that she may be having a phantom pregnancy? She may well be feeling a lttle strange and uncomfortable?

that is worrying but i wouldn't panic. i'd consider a possible physical issue. hopw things get sorted X

A phantom pregnancy did cross my mind. What are the signs I should look out for? Apart from this aggression she has also seemed a bit fed up the past week, either snuggled up with me or OH on sofa or completely takes herself away from us all. Would you recommend a trip to the vets?

i agree with the phantom, however if this persists, try a recognised behaviorist, or along that lines, the vet would be able to tell if its a phantom, you could check if her nipples are swolen and our bitch is sometimes off her food, hope you get sorted,

sami x

Thanks for your replies. She is definately not off her food and her nipples dont appear swollen but I will keep an eye on things. I was planning to contact the trainer whos puppy class we took her to as she was really good and helped us a lot. Thought I would take some advice from here though first.
Its really got me worried though that I can't trust her when I used to be able to. Also don't want her to start on my other dog or for him to copy as he is so calm and laid back. She has always been the more unruly but as I mentioned in my first post has never acted aggressively before.

Sorry to hear about your trouble with your lass .. Maybe she is being a bit off because  of what others have said or maybe she is in other dicomfort and can't let you know. If she is generally a good natured dog and this is out of chararcter for her there may be something troubling her..I have one husky more unruly than the other , but she has never showed any aggression towards people or dogs , except for once she gave a dog, we meet often, a good nip for sniffing round her back end and it turned out she was having a silent season .....

UPDATE
Thanks for all your replies. We haven't met any other dogs on our walks until today when we met a lovely little bull terrier around the same age as our bitch who we have met before on the odd occasion. I walked towards them with our boy (who is not the problem) whilst OH stayed slightly behind with Missy and made her sit. I mentioned to the couple with the dog that Missy had been a bit 'funny' towards other dogs and thats why OH hadn't come over. Anyway, they were fine with this, walked over to Missy who sniffed, tail slightly wagging (as before when she had the 'fight') but then let out a growl. Quick tug on the lead and sharp 'no' from OH and she stopped and was then fine with other dog and all 3 had a play together.
As I mentioned on my original post it is all noise - no biting, baring teeth so am now thinking it might be a dominance issue (seen as shes teenage equivalent) and is trying to intimidate other dogs with noise to show them whos 'boss'. If this is the case though I still want to get this sorted as many dogs will not take too kindly to this!

Good to hear ... Had to laugh at the "trying to intimidate other dogs with noise to show them whos 'boss'. " one of mine used to leap off the ground , all four paws off the ground , with her head at my eye level , to prove she was the boss ...other dogs and their owners were not impressed ..But she so stopped when she eventually realised it got her nowhere ... Huskies are dafties sometimes...

Glad mines not the only daft one!