Is it normal ?

So I am 24 weeks am having a boy and last checkup my midwife told me my belly was measuring bigger . She said no biggie . Now I just went to do my 24 week follow up and she said the same thing . But this time I actually asked what that meant and she said my belly should be measuring 20 - 24 weeks ( how ever the measure it by ) but am measuring at 27 ! She said she will order a ultrasound in a month and go from there ... I have only gained 4 lbs in my overall pregnancy. Has this happened to anyone else ? And if so what does that mean for me ??