Seriously got no clue...

So my ob's hospital called me yesterday to schedule my 21 week ultrasound and also informed me that they are making me speak to a dietitian. I am already bugged by my dr being kind of ugly about the fact i have to get a glucose test done at 19 or 20 weeks... i have no idea what to think about it all and have been severely depressed by it. I will be 17 weeks this Sunday and have only gained 1 lb after losing about 5 to start with. My husband isn't being as supportive and just tells me it's fine. Anyone with advice i would love to hear it and as for an actual question why do i have to go to the hospital for this ultrasound?? My dr has a machine in her office and I've seen the baby before multiple times. I am totally clueless. Thank you all in advance.