Measuring two weeks behind

I went to my midwife appointment today where she measured me for the first time, she also weighed me I was 42kg but now I'm 53.4kg so I have put on weight, I'm short and skinny but have a low bmi, anyway when she measured my belly I should of been 27 weeks ( I'm 27 weeks 4 days) but I only measured at 25 weeks and she said baby was small, I have a growth scan Monday at 28 weeks but baby had a good heartbeat, FTM, has anyone else experienced this? Will I just have a healthy smaller baby?