Weight Gain

I am a FTM and 20 weeks. Before I got pregnant I weighed 142 which was pretty normal for me since I'm 5'6". In my first trimester I had had a hard time eating so by 12 weeks I weighed 133. I hung out around 133 for a few weeks until I finally started gaining weight at about 15 weeks. I'm now about 137 pounds.

According to all the pregnancy weight calculators I should weigh around 150-155. Baby measured right on schedule at my 14 weeks ultrasound but I'm still worried that I haven't really gained anything. I guess I could use some insight and reassurance. I have my anatomy scan tomorrow