Not showing still when I am in my seventh month of pregnancy

Hi moms

I am a little concerned because I am not showing nor gaining weight and I am in my seventh month/ third trimester of pregnancy now. I understand everyone is different but I am to the point where I am worried that the baby will have growth restrictions when I go back to have my next ultrasound. My doctor saw a healthy baby in the ultrasound but put me on an every two week schedule for appointment before my third trimester started. My pre pregnancy weight was on the very low side and I still have a low bmi of 18 and I am considered petite in height. Until now is only 5 pounds gain and I am fiting in to size 25 clothing still. I measure my belly at home and I'm not noticing growth but I feel my baby moving constantly especially after meals, stretching, or from certain sounds like baking of the dogs. Did anyone else not show/gain weight until really really late into pregnancy. I have tried everything to help gain weight protein powders and eating small meals more frequently. As my pregnancy has progressed I definitely have been having gastrointestinal issues. Thank for the advice and support in advanced!