Measuring Smaller than expected

I've just had my 34 week app with the midwife and she mentioned that I'm measuring around 3inch(cm?) Smaller than expected. She mentioned that it can be between 3 and 3 1/2 either way before they send you for scan and that she wants to send me for scan for measurements and fluid check etc. At my last app I had reduced movement and was sent to the hospital for heart trace. Has any1 else had this experience? My baby has been very active since the heart trace, and i feel huge however I'm very worried now as I haven't been taking my pre-natal vitamins regularly.