aggressive behaviour

[k.harding]

I'll start with the full story sit down its gonna be quite the read.
We got our dog (collie cross) in november 2005 from the rspca she was around 2 years old and had been there for around 9 months, we then got a kitten around november 2006 and after a week the kitten got playfull and the dog played too roungh and hurt the kitten, this was by accident and the only incident up and till now, the dog had growled at the cat when ever he tried to go near her food but thats it, This April we moved to canada and brought them with us we stayed at my brother inlaws house who had 3 dogs and they didnt get on with each other so we kept them separated we stayed there for 3-4 months in that time our dog seemed to have a hate on for the eldest dog there and if she got the chance went straight for him, and 3 times managed it and was very nasty, allthough never went for any of us, we then moved in to our own place and took over my other brother inlaws cat who is around 6 years old, this cat simply didnt like poppy and kept away or would hiss when our dog walked by, our dog on the other hand didnt seem to care less, but a week ago our dog went for the new cat and bit him, and tonight went for our other cat and bit him too, there are a few other details the animals have recently gotten fleas, we treated the dog with hertz one spot about 3 weeks ago, and the swith advantage, and both incidents my wife had finished flea combing the dog who doesnt like been brushed at the best of times, the first time we though maybe a act of dominance, but now shes gone for our other cat we are stumped especially since they used to play together all the time and sleep together etc, we really want to get to the bottom of this because we love all the pets but we would be irresponcible to let this happen again, especially since our dog has such a nice temprament and is very affectionate.