Sue, being that this is not your body's first pregnancy, your uterus may simply be used to stretching quite quickly...anyhow this is how my nurse friend who is mother of three explained to me why my belly seemed to have popped so early.
If you dont feel comfy buying 'maternity' clothing....why not try some items on that are one size bigger than your normal clothes.

I know that jeans west stock comfy jeans in half sizes...so I actually bought 2 pairs for $90 in a size 15 and they are still quite comfy, and will be great to wear after bubs is born.