Measuring 5 weeks behind

At my last three appointments I've measured behind. The first two was 2-3 weeks behind and today I measured nearly 5 weeks behind. Measured 27 when I'm 31+5. My midwife said this is the point where she would order an ultrasound but I already have one Friday to check on my low lying placenta, so they will check it on her size then.

This is my first pregnancy so I really don't have anything to go by but she feels so strong and I feel myself getting bigger and more uncomfortable, it's hard imagine she's undersized. I'm trying not to worry but I've been a vegetarian for 10 years and I haven't been able to stomach the prenatal vitamin so at my midwives recommendation I've been taking folic acid and a children's vitamin. I feel terrible to think it hasn't been enough for her.

Has anyone else had this happen? Did you have a growth scan? What was your outcome?