It's fine.. Everyone carries their baby differently.. Since this is your first child (I'm assuming) it just might take longer to show.. I didn't start showing with my first until my 7th month. You will show, it just might take a lil while!! Good luck!!
In my first pregnancy, my stomach also was small and my hips and bottom grew larger too. I had a girl, :-) For my current pregnancy, I cannot believe how large my stomach has gotten, and while the rest of my body has grown too, I am smaller in other places this time around.
I wouldn't worry too much. Eat, drink, and be merry, :-)
Around 24 weeks the doctors will start to measure your belly. They measure from the top of your belly near your bust line to your pubic bone. It is measured in centimeters and you should measure the number of weeks that you are, give or take 2 cm. For example, I am 26 weeks and I should measure 26cm, or 24-28cm when you account for the +/- 2cm.
Haha! I bet you're having a girl. That butt/thigh combo is SO girl, at least for me.
Are you long-waisted or tall? I have a friend who had an 8lb baby (was about 115 lbs, 5'8" before pregnancy) at term who never had to wear maternity pants, because she carried high and inside. It really depends upon your body type. I was about 5'3", 150lbs for my first baby and was in maternity clothes by 12 wks and showing.