Low Amniotic Fluid and confused?

I am very confused right now. I go to the doctor for another ultrasound tomorrow but until then I would like to see if anyone knows anything about this. I was put into the hospital a few weeks ago for a funneling cervix (incompetent cervix) and got meds to stop progression. I was 25 weeks. Now Im 29 weeks and for the past 5 days had been sick and got dehydrated. Went to the doctor and I was measuring 32 weeks instead of 29.. So they did an ultrasound to check the fluid and I was expecting them to say that I had more fluid than I should, but they said the amniotic fluid level was slightly lower than it should be, but not dangerous. The doctor told me that it is because I was dehydrated and needed to drink gallons of water before my ultrasound that will be done again tomorrow. She says if the amniotic fluid levels are not back up she would stick me in the hospital. Im confused because everything I have read about measuring larger indicates more amniotic fluid, and I cant find.........