I wouldn't worry too much until you know more. The ultrasound should clear things up for you! Stress doesn't help you or your baby.
Even if your baby is actually 33 weeks, not taking your prenatals throughout your whole pregnancy wouldn't change anything at 33 weeks. (At least I wouldn't think it would). You may have just conceived early which is why your baby is only measuring 33 weeks.
These are just guesses... I'm not a doctor, so just try to relax until the ultrasound! I'll be praying for you and your baby!