No gaining the weight. Should I be concerned?

Questions for fellow mommies:

So about 2 weeks ago I went in for my prenatal appointment. I was concerned because I didn't reach my goal wait. Yet when the nurse weighed me she weighed me 4 pounds heavier then I actually was. She used the old school weight scale machine and I could tell she didn't know how to use it. So my doctor said I was fine. Then I figured maybe my scale at home was wrong.

Flash forward to another doctors appointment where they tell me I the same weight I that I weighed myself on my home scale and say I'm under weight and if it continues to be a concern check back with them.

Now I'm 24 weeks pregnant and I should be somewhere between 140-146. I am only at 137 meaning Iv'e only gained 7 pounds this entire pregnancy. Pretty much only gained one pound since my doctor appointments. Am I okay or should I be concerned? Thought I might ask friends before calling the doctor.