In my current experience, it's not just the new baby, but the age too. My well-mannered, easy going little girl has become defiant, moody and at times, outright revolting over the past few weeks, just as she's ticked over into 3.5. When lamenting about it, I've had quite a few parents say that they saw a deterioration in behaviour at this age too.

I also found this. http://planningwithkids.com/2009/11/...r-old-behavior really interesting

I'm honestly finding it really hard to keep my cool... part if me immediately feels shocked and indignant. How DARE she talk to me like that? She's been so 'good', so she knows better.... BUT...underneath the infuriation, I know better than that.
It's helping to remember that she's only three. That her behaviour comes from a place of anxiety, confusion, a need for boundaries, experimentation... not from any place of malice or intent t make my life difficult.

I'm finding that as soon as I find myself getting angry, I have to stop myself, breathe, give myself a stern talking to to not fight with a child... and then try the following (depending on what I think the situation calls for)
- be silly and help her diffuse and laugh it off
-offer simple choices (do you want my help to calm down, or do you want to go and be cranky over there?)
- reflect her emotion and the reasons for them
- shrug and say 'ok then... I can see you're cross. That's ok, I feel cross sometimes too.' And then go on with what needs to be done.

I find that she settles much faster if I'm just unfased by her outburst. Janet Landsbury http://www.janetlansbury.com/ also has some great posts on managing defiance and tantrums. I loved a point she made that emotionally reacting to a child's outburst gives them too much power. More than they actuall want or can cope with. If we show them that it's ok to have big emotions, we're fine with that and we're going to be clear, firm and maintain the boundaries (and the love) regardless of what they do... that makes them safe an helps them move through this stage.

Hope some/any of that helped. It helped me writing it down anyway as oooohhh it's hard to be unfased when being bossed/shouted at/ ignored/ hit after a long day.

Good luck.xox