My best friend was the SAME way!! Even at her delivery, she barely looked pregnant.
her first son was 6.13, and her 2nd was 5.15. She had them at 38 weeks.
She was SUUUPER tall, and Super thin, so you couldn't see a bump until she was about to pop!! And even then she was tiny!!
Don't worry! Count yourself as lucky!! LOL!
Sarah