Batha, I too didn't want children - in fact could not imagine anything worse: Career-wrecking, social life-imparing, screaming, joyless exercise.
Then this year, aged 36, I conceived by accident after a one night stand. I had to do a LOT of thinking. About how my life would be in 10, 20 years with or without children, whether I would be strong enough to raise a child on my own for the next 18 years (if it came down to that), whether I would resent it for 'ruining my life', or actually, whether it would bring something extra to my existence.
I decided to go ahead. It's not been a bed of roses, in fact the worst thing is still the unknown - will I have a baby like your brother who 'screamed for four years' or will I have a very laid-back baby? Who knows... ultimately, it's the hardest decision to make when the variables are completely random.
What I'm saying is I never had an overt desire for children until it happened. I think that is quite normal. It's also completely normal to not want to have children. Sometimes pets are a great alternative!
Can you talk to your husband about where this change-of-mind has come from? Perhaps you could use some form of mediation (ie couples counselling) to find out more about where you both stand?