Hey Ally, I felt really bloated straight away and by 10 weeks I was heaps bigger too. I was told that it's because of the extra fluid you're carrying.
Also, it looks like you've had a son already...? I've heard that when you get pg the second and subsequent times your tummy muscles say, 'hey! I've done this before!' and they get bigger quicker... Not sure if that's the same for everyone or not...
Best wishes for the rest of your pregnancy!