Weight concern

So im 19 and 4. Baby number 3. At the begining of pregnancy i lost about 20 lbs (i wasnt throwing up, just nothing was appetizing) and I havent gained an ounce back. This pregnancy is extremely welcomed and i am so excited, but our circustances have been extremely stressful. Im a bit concerned the baby could not be gaining enough weight and will come out small (this happened with my first, but not my second) any thought or suggestions would be great