3rd trimester and weight

I’m in my 3rd trimester and my weight has been up and down (mainly down) but I’ve been under my pre-pregnancy weight since week 8. I’m concerned because everyone says you’re supposed to gain weight in your last trimester. Is this something I should worry about? She’s also measuring smaller which my mom says I shouldn’t worry because not all babies are the same but this is my first pregnancy so I am kind of worried.