Belly measures small at 33 weeks

Rachel • 25 years of age -- one angel baby (Emelia Rose) and my rainbow baby born 07/13/15 (Layla Marie). Baby boy due 03/03/2017
I went to the doctor yesterday (Thursday) and the doctor said my belly measured 31cm, which is spot on for 31 weeks. My issue is is that I'm now 33 weeks. The doctor I saw that day reviewed my chart and said my baby is trending smaller since I've been measuring a cm behind each week. I know my due date is correct because it was confirmed at my anatomy scan at 20 weeks. Has anybody else's belly measured small for their current week? I'm a tad concerned, but she said not to worry unless she was measuring 3 behind. What's the cause of me measuring so small? Is my baby going to be smaller or is there something wrong?? Help! I'm worried to tears since I've already lost one baby! This is my rainbow baby after losing one at 18 weeks. 😢