Feel like I’m too small 😩

I’m 31 weeks and 3 days and I feel like I’m way too small. When I’m standing up I feel like I have a bigger belly but it’s not firm like a belly should be & laying down it seems like it disappears! Just makes me worry he isn’t growing right especially since I haven’t had an ultrasound since 20 weeks and he was in the 40th percentile and last fundal height I was under but still within range but that was 6 weeks ago... ive also only gained about 15 pounds. I ’m sure I’m overthinking but any words of advice or thoughts would be appreciated!!