Could be baby was in a different position. That happened to me with my son but I was already having an ultrasound every week so they just measured my fluids when I had my next one. Everything was fine. Sometimes the measurements are iffy too if the doctor doesn't start from the exact same place every time. That happened this time with my daughter. I see a rotation of doctors and they all seem to start higher or lower than the next did so my measurements were always a little different depending on who did it.