Weight concerns during 3rd Trimester

I’m at FTM and today, 28 weeks! I’m 20 and a naturally slender person with a high metabolism. It’s always been hard for me to gain weight. In high school I weighed 107. I went to college, became pescatarian, and lost my muscle weight due to ceasing my athletic routine. My body fat percentage was always really low when they checked it during track practice. (Muscle weighs more than fat) I went down to 94. In my second trimester, I only went up to about 106. I haven’t gained any weight since. I eat when I’m hungry, and to me and my fiancé it’s a lot! Should I be concerned? Is something wrong?! How do I gain more weight?