Baby Measuring Small?? Advice please

I am 31w and today I met my first meeting with my midwife. My doctor was really late on things and I haven't met them until now so it's a bit behind but we are all caught up now apart from this 1 problem.

All my ultrasounds are clear like she's growing healthy on the inside and have no abnormalities on the outside, but today at my meeting she has measured my stomach at instead of measuring at 31, she is measuring 28.

The midwife said it's OK, I shouldn't be stressed and she wasn't stressed about it but she's booked me into a ultrasound next week to check her growth, like find out if it's something to do with my body or anything.

My partner and I are now so stressed. I don't do anything bad, like the smoke alarm went off from the toast burning and my partner freaked out like "Not good for the babies ears!!!"

I've had a great healthy pregnancy, but I didn't know I was pregnant until 13 weeks and up until that point I was smoking + drinking, and that has me even more stressed out like could I have hurt her in those 13 weeks ? (Midwife knows of the smoking etc)

I haven't touched anything since week 13 but I'm really scared.

Could something be wrong with her?

Is it likely that she's just a small baby?

I am looking for advice from anyone who've had small babies, I don't expect anyone to know true answers as none of your are my doctor but any advice would be helpful. Did you have a small baby? Was she okay or did you have a medical issue?

It's only 1 week until this scan but it's very scary in the mean time.