You're measuring one week behind at 36 weeks? Hello, what's the big deal? Have you been measuring dead on all along? Was there a significant drop? (more than 4 weeks?) Fundal height is such a subjective thing, it's really hard to even consider it something to base any medical information on. He could have measured starting from a different place. Someone else could have measured you one week, and then someone else the next. At 32 weeks, one person measured me at 38 weeks, and had a fit about it, then four weeks later, at 36 weeks, another person measured me at 37 weeks. Do you really think that it's possible that I was 1 cm smaller in a whole entire month? My clothes sure have a different tale to tell. I'd ask him for some evidence that this is cause for alarm. You're also pretty far along, and it's entirely possible that the baby is dropping, which could account for smaller measurements as well.
With the white blood cells, were you particularly hot shortly before you left your sample? I spilled some white blood cells at one appt, but I was burning up in this Florida heat, and it's been dismissed now as just a heat response (after looking at a vaginal swab under a microscope). I wouldn't worry about it, provided the number is small, and doesn't increase.
As for the fluids, it's always a good idea to drink a lot of fluids while you're pregnant. How much are you drinking? One formula I've heard is to take your body weight, divide by 2 and aim for that many ounces. Others I've heard say flat out drink a gallon of water a day. Somewhere in between is probably a good spot to land.
Good luck, and don't let them stress you out. Have them show you the evidence about amniotic fluid, and relax.