Measuring small

So I am 35 weeks today at my Drs visit she measured me and said my son is measuring only 33 . His heart rate is fine and his movements are too . She wants to do a ultrasound Friday and wants me drinking protein shakes until then . Oh yea I haven't gained a pound this whole pregnancy. Not one . I eat all day everyday junk food and fruits and veggies . I'm so scared that something will be wrong and cannot think straight . Please keep me and my son in your thoughts and prayers 🙏🏾👶🏽