Bubs have growth spurts inside and outside the womb - maybe yours was just about due for one when the last u/s was taken.

The first 12w most healthy babies will grow at teh same rate (therefore size measurements at this stage are fairly accurate regarding due dates) but after that the growth rates vary and they have spurts every now and then.

The measurements are based on all babies growing at the same rate, and relies on them growing at that rate consistently throughout the pregnancy. That's why it should be used as just a guide as to A) when the baby is due and B) how big it will be when it's born.

It sounds like everything is going swimmingly in there :P So please try not to worry.