Weight problems

So I went to the doctor on February 9th and I am scheduled to go back for an ultrasound and check up on the 23rd of this month. .. but my doctor has been talking a lot to me about my weight he says I am not gaining enough weight I have only gained 3 pounds since Christmas and only about 8 pounds my whole pregnancy. . Is anyone else struggling? I feel like I am failing my baby of proper stuff it needs by not gaining weight ... but I just can't eat a lot I never feel hungry and even when I can eat I can't eat much ..