Too little at 30 weeks?

I am a first time mother and I constantly get told I am way too small and my belly should be bigger. It has me worried why can’t I gain weight or keep it up. I have always had a fast metabolism my whole life and always been skinny. My pre-pregnancy weight is 120. I am now 30 weeks and weigh 135 & I am 5’6. I’m just worried and emotional. My doctor doesn’t seem to ease my mind either when I ask they say it fine when I feel

it’s not!