measuring behind?

so i’m 34 weeks. every 2 week appt my midwife has told me i’m measuring EXACT and perfect. today i had an appt and my midwife wasn’t in so they had me see some new person who just started working there and she said i’m “measuring a tiny bit behind” and they’ve scheduled me for a ultra sound in 3 days. she said she has a strong heartbeat & it could simply be normal because i’m tiny and petite. but now she made me worried. i don’t understand how i was measuring perfect and now all of a sudden i’m a tiny bit behind🤔