30 weeks + no weight gain?

Just curious, here. And a little concerned. I am 30 weeks exactly today, actually. Let me start from the beginning. I have one child already and began that pregnancy at 5'7"  and 138lbs. I ended that pregnancy at 172 and after birth I stayed a consistent 155 until I got pregnant 1 1/2 years later. This time around, I started at 155, dropped down to 145 in the first trimester due to feeling nauseous constantly and having food aversions altogether. Throughout the second trimester up until about 24 weeks I bumped up to 155 slowly and have since plateaued. I go back and forth between 154 and 156, but I'm just not gaining the way I feel I should. My doctor has mentioned it several times but doesn't seem to be doing anything about it. I'm eating when I'm hungry and good amounts, a little healthier and definitely not overeating like I did with my first pregnancy but am by no means starving myself. My belly is much smaller than my first time around, as well. Has anybody had this issue and had a healthy baby? Should I look into a different doctor? I haven't had an ultrasound since 20 weeks and it was literally a 5-6 minute ultrasound so I feel they couldn't possibly have checked for problems that quick.